question: Why do American writing conventions (Chicago, MLA, etc.) mandate that periods and commas directly following quotations should lie within them?
comment: It's all just convention, there's no reason behind any of it. A publisher has that convention and they gotta stick to it, and they need to have a standard, any standard, or else it will be impossible to properly copy edit. If you publish a journal or edited volume and everyone uses random conventions and standards that will be very ugly and distracting. And if you don't have a convention then in a single article/book how else would you make it consistent without a set of rules. And it's too late, they won't change their conventions or have a unified convention.
passages: When dealing with direct speech, British placement depends on whether or not the quoted statement is complete or a fragment. According to the British style guide "Butcher's Copy-editing", American style should be used when writing fiction. Periods and commas that part of the person's speech are permitted inside the quotation marks regardless of whether the material is fiction. In non-fiction, some British publishers may permit placing punctuation that is not part of the person's speech inside the quotation marks but prefer that it be placed outside. Periods and commas that part of the person's speech are permitted inside the quotation marks regardless of whether the material is fiction.
   However, some American style guides specific to certain specialties, such as linguistics, prefer the British style. For example, the journal "Language" of the Linguistic Society of America requires that the closing quotation mark precede the period or comma unless that period or comma is "a necessary part of the quoted matter". When dealing with words-as-words, short-form works and sentence fragments, standard American style places periods and commas inside the quotation marks:
  
  This style also places periods and commas inside the quotation marks when dealing with direct speech, regardless of whether the work is fiction or non-fiction:
   Nevertheless, many American style guides explicitly permit periods and commas outside the quotation marks when the presence of the punctuation mark inside the quotation marks will lead to ambiguity, such as when describing keyboard input:
  
  Section::::Order of punctuation.:Ending the sentence.
  
  In both major styles, regardless of placement, only one end mark (?, !, or .) can end a sentence. Only the period, however, "may not" end a quoted sentence when it does not also end the enclosing sentence, except for literal text:
  
  /ref
   BULLET::::- In typography, for example, some customs persist, although the reason for their existence no longer applies; for example, the placement of the period inside a quotation in U.S. spelling. In metal type, pieces of terminal punctuation, such as the comma and period, are comparatively small and delicate (as they must be x-height for proper kerning.) Placing the full-height quotation mark on the outside protected the smaller cast metal sort from damage if the word needed to be moved around within or between lines. This would be done even if the period did not belong to the text being quoted.
   Quotation marks are used to mark quotation. In all forms of English, question marks and exclamation points are placed either inside or outside the quotation marks depending on whether they apply to the whole sentence or only to the quoted material. In British publications (and those throughout the Commonwealth of Nations more broadly), periods and commas are most often treated the same way, but usage varies widely. In American publications, periods and commas are usually placed inside the quotation marks regardless. The American system, also known as "typographer's quotation", is also common in Canada, and in fiction publishing broadly.
   In the United States, the prevailing style is called American style, whereby commas and periods are almost always placed inside closing quotation marks. The American style is used by most American newspapers, publishing houses and style guides in the United States, and to a lesser extent, Canada as well. It is the style usually recommended by the Modern Language Association's "MLA Style Manual", the American Psychological Association's "APA Publication Manual", the University of Chicago's "The Chicago Manual of Style", the American Institute of Physics's "AIP Style Manual", the American Medical Association's "AMA Manual of Style", the American Political Science Association's "APSA Style Manual", the Associated Press' "The AP Guide to Punctuation" and the Canadian Public Works' "The Canadian Style".
   When dealing with words-as-words, short-form works and sentence fragments, this style places periods and commas outside the quotation marks:
   "Hart's Rules" and the "Oxford Dictionary for Writers and Editors" call the British style "new" quoting. It is also similar to the use of quotation marks in many other languages (including Portuguese, Spanish, French, Italian, Catalan, Dutch and German). A few US professional societies whose professions frequently employ various non-word characters, such as chemistry and computer programming, use the British form in their style guides (see "ACS Style Guide"). According to the "Jargon File", American hackers switched to what they later discovered to be the British quotation system because placing a period inside a quotation mark can change the meaning of data strings that are meant to be typed character-for-character. (It may be noted that the current American system places periods and commas outside the quotation marks in these cases anyway.)
   In citing an act by its short title, a comma immediately before a reference to a year and a comma immediately after such a reference that is not required for the purpose of punctuation may be omitted.
  
  Section::::Definite article.
  
  The Australian Guide to Legal Citation recommends that the definite article at the beginning of the "statute title" should be omitted when citing a statute of the United Kingdom.
  
  Section::::Interpretation.
  
  In "Re Boaler", Buckley LJ said:
  
  In "R v Wheatley", Bridge LJ said of the Explosives Act 1875 and the Explosive Substances Act 1883:
  
  Section::::History.
   Many publishers have established an internal house style specifying preferred spellings and grammatical forms, such as serial commas, how to write acronyms, and various awkward expressions to avoid. Most of these are internal documentation for the publisher's staff, though various newspapers, universities, and other organizations have made theirs available for public inspection, and sometimes even sell them as books, e.g. "The New York Times Manual of Style and Usage" and "The Economist Style Guide".
   Some writers precede quoted material that is the grammatical object of an active verb of speaking or writing with a comma, as in "Mr. Kershner says, "You should know how to use a comma."" Quotations that follow and support an assertion are often preceded by a colon rather than a comma.
  
  Other writers do not put a comma before quotations unless one would occur anyway. Thus they would write "Mr. Kershner says "You should know how to use a comma.""
  
  Section::::Uses in English.:In dates.
  
  Section::::Uses in English.:In dates.:Month day, year.
   As noted above, in some older texts, the quotation mark is repeated every line, rather than every paragraph. The Spanish convention uses closing quotation marks at the beginning of all subsequent paragraphs beyond the first.
  
  When quoted text is interrupted, such as with the phrase "he said", a closing quotation mark is used before the interruption, and an opening quotation mark after. Commas are also often used before and after the interruption, more often for quotations of speech than for quotations of text:
   According to the Associated Press, the ellipsis should be used to condense quotations. It is less commonly used to indicate a pause in speech or an unfinished thought or to separate items in material such as show business gossip. The stylebook indicates that if the shortened sentence before the mark can stand as a sentence, it should do so, with an ellipsis placed after the period or other ending punctuation. When material is omitted at the end of a paragraph and also immediately following it, an ellipsis goes both at the end of that paragraph and at the beginning of the next, according to this style.
   But lists can also be written in other ways that eliminate the ambiguity without introducing the serial comma, such as by changing the word order or by using other punctuation, or none, to introduce or delimit them (though the emphasis may thereby be changed):
  
  An example collected by Nielsen Hayden was found in a newspaper account of a documentary about Merle Haggard:
  
  A serial comma following "Kris Kristofferson" would help prevent this being understood as Kris Kristofferson and Robert Duvall being the ex-wives in question.
  
  Another example is:
   There are a few differences in punctuation rules. British English is more tolerant of run-on sentences, called "comma splices" in American English, and American English requires that periods and commas be placed inside closing quotation marks even in cases in which British rules would place them outside. American English also favors the double quotation mark ("like this") over single ('as here').
   Direct quotations are introduced with a comma followed by the utterance in double quotes. The comma is used to indicate the pause which is mandatory in speech when introducing quotes, and indeed, in older orthographies the quotes were not used at all since the pause by itself is sufficient to introduce the next phrase as a quotation.
   Expectations of style and format are influential in the format and development of a professional document. Precedents created by earlier documents of the same genre of a professional writer's work heavily influence how the reader of his or her document will judge the credibility of both the writer and document. Documents belonging to a specific genre are expected to be written in a way that adheres to a format and style that defines that genre. If a professional writer were to produce a document that does not adhere to the precedence style, he or she and the document would lose credibility.
   The style that always uses the serial comma may be less likely to result in ambiguity. Consider this apocryphal book dedication:
  
  There is ambiguity about the writer's parentage, because "Ayn Rand and God" can be read as in apposition to "my parents", leading the reader to believe that the writer claims Ayn Rand and God are the parents. A comma before "and" removes the ambiguity:
   The comma preceding the calendar year in printed copies of acts is omitted on the authority of a note by Sir Noel Hutton QC, First Parliamentary Counsel, as to which see "The Citation of Statutes" 82 LQR 24-24. The validity of this note is questioned by Halsbury's Laws of England, Fourth Edition, Reissue, Volume 44(1), footnote 10 to paragraph 1268.
  
  Glanville Williams said that it "seems sensible" to omit the comma preceding the calendar year in references to acts passed before 1963.
  
  Section::::Comma.:Ireland.
   Journalist Oliver Kamm writes of novelist Jane Austen's use of the comma splice, "Tastes in punctuation are not constant. It makes no sense to accuse Jane Austen of incorrect use of the comma, as no one would have levelled this charge against her at the time. Her conventions of usage were not ours".
   and the "AP Stylebook",
  
  recommend that the second element be treated as a parenthetical, requiring a second comma after: ""The plane landed in Kampala, Uganda, that evening.""
  
  The United States Postal Service and Royal Mail recommend leaving out punctuation when writing addresses on actual letters and packages, as the marks hinder optical character recognition.
  
  Section::::Uses in English.:In numbers.
   Section::::Usage.:Use–mention distinction.:In linguistics.
  
  Precise writing about language often uses italics for the word itself and single quotation marks for a gloss, with the two often not separated by a comma or other punctuation, and with strictly logical quotation around the gloss – extraneous terminal punctuation "outside" the quotation marks – even in North American publications, which might otherwise prefer them inside:
  
  Section::::Usage.:Titles of artistic works.
   The convention used to be, and in American English still is, to put full stops (periods) and commas inside the quotation marks, irrespective of the sense. British English has moved away from this style while American English has kept it. British style now prefers to punctuate according to the sense, in which punctuation marks only appear inside quotation marks if they were there in the original. Formal British English practice requires a full stop to be put inside the quotation marks if the quoted item is a full sentence that ends where the main sentence ends, but it is common to see the stop outside the ending quotation marks.
   With narration of direct speech, both styles retain punctuation inside the quotation marks, with a full stop changing into a comma if followed by explanatory text, also known as a dialogue tag. Americans tend to apply quotations when signifying doubt of veracity (sarcastically or seriously), to imply another meaning to a word or to imply a cynical take on a paraphrased quotation, without punctuation at all.
  
  Section::::Typographical considerations.
  
  Section::::Typographical considerations.:Single nested within double, or vice versa.
   There is no space on the internal side of quote marks, with the exception of ¼ "firet" (~ ¼ em) space between two quotation marks when there are no other characters between them (e.g. ,„ and ’”).
  
  The above rules have not changed since at least the previous BN-76/7440-02 standard from 1976 and are probably much older.
  
  However, the part of the rules that concerns the use of guillemets conflicts with the Polish punctuation standard as given by dictionaries, including the "Wielki Słownik Ortograficzny PWN" recommended by the Polish Language Council. The PWN rules state:
  