Why are there so many translations of the Bible? It seems confusing to have so many translations of the Bible.  Why isn't there just a single (English) one?  Or maybe a single "modern" one (given that KJV is hard to understand these days).
Being a speaker of a minority language, I can state one good reason and many -- well, not so good ones.
The good reason
Language changes. In some languages, you only have different Bible translations for this reason: the translations are temporally spaced maybe a hundred years apart.
Look at 2 Timothy 2:15 in different English translations:

The Wycliffe Bible, 1395
Bisili kepe to yyue thi silf a preued preisable werkman to God, with oute schame, riytli tretinge the word of treuthe.
Tyndale New Testament, 1526
Study to shewe thy silfe laudable vnto god a workman yt nedeth not to be a shamed dividynge the worde of trueth iustly
King James Version, 1611
Studie to shewe thy selfe approued vnto God, a workman that needeth not to be ashamed, rightly diuiding the word of trueth.
King James Version, 1769
Study to shew thyself approved unto God, a workman that needeth not to be ashamed, rightly dividing the word of truth.
Young's Literal Translation, 1862
be diligent to present thyself approved to God -- a workman irreproachable, rightly dividing the word of the truth;
The New American Standard Bible, 1995
Be diligent to present yourself approved to God as a workman who does not need to be ashamed, accurately handling the word of truth.

It's quite necessary to renew the Bible translation time to time, so that people can understand it. The 1769 KJV is still quite understandable for the most part, but the reasons are cultural. The huge popularity of the KJV has kept its language alive.
The not-so-good reasons
Translation philosophy
There are other reasons, which vary from good to bad in different instances. First, there are the different translation philosophies already discussed in another question -- this amounts to multiple reasons.  Note that there are benefits to having different types of translations.
Doctrinal reasons
Then there are the doctrinal reasons -- in my opinion, bad reasons. I'll show just one example, the New World Translation's take on John 8:58.

New World Translation
Jesus said to them: “Most truly I say to YOU, Before Abraham came into existence, I have been.”
New American Standard Bible
Jesus said to them, &quot;Truly, truly, I say to you, before Abraham was born, I am.&quot;

Looking at other English translations of John 8:58, every one of them translates the end as &quot;I am.&quot; Looking at an interlinear version, one can be quite sure that the NWT translates the passage on a purely doctrinal basis.

What's very interesting is that the Watchtower Society has an interlinear of their own, and even that translates the phrase as &quot;I am&quot; while also showing the NWT &quot;I have been&quot;:

The bad reason turned good reason
If the NWT was the only available translation in English, it would almost force a new translation to be made. Take a bunch of errors such as the one shown above, and the translation gets too unreliable.
Of course, the errors need not be especially crafted to suit some doctrine; they can be made of ignorance, disbelief in the original, etc. First translations in a new language (e.g. what the Wycliffe Bible Translators are doing) almost necessarily contain errors, as many needed terms are very hard to find for a non-native speaker. Whatever the reasons are, a large amount of errors is a good reason to create a new translation.