Why does Christmas end on the Baptism of the Lord and not on the Presentation of the Lord? The Presentation of the Lord is celebrated exactly 40 days after Christmas, yet Christmas ends on the Baptism of the Lord, which is the beginning of Ordinary Time.

Why is the Christmas season short when compared to the Easter season, which is celebrated for 50 days? How was it decided that the day celebrating the Lord's baptism marked the end of Christmas?
Why does Christmas end on the Baptism of the Lord and not on the Presentation of the Lord?

The Christmas season still ends on the Feast of the Presentation of the Lord. Some so called liturgical calendars seem to state that it ends with the Feast of the Baptism of Our Lord, but this is not congruent with the Church's sacred tradition.

The Christmas Season starts with First Vespers of the Nativity of the Lord and continues with a holy joy until the 2nd Vespers of the feast of the presentation of the Lord (February 2nd).The celebration of a 40 day season for Christmas goes so far back into antiquity that it is impossible to know at what point in time it became a tradition. The Feast of the Presentation was established as a feast on this date at least by the 6th century. Mary was obliged by Jewish law to present Our Lord to the Lord 40 days after his birth, thus bringing the joys of Christmas to a natural conclusion.


  Until 1969, the ancient Feast of the Presentation of Our Lord, which is of Oriental origin, was known in the West as the feast of the Purification of Our Lady, and closed the Christmas season, forty days after the Lord's birth.- Directory on Popular Piety and the Liturgy (120) of the Vatican Website.


Traditionally speaking, our Christmas trees should go up on Christmas Eve and brought down on the Feast of the Epiphany with the arrival of the Magi with their gifts of gold ,frankincense and myrrh. On this day the three wise men are added to our manger scenes! In Europe, the Christmas Crèches are only taken down on February 2nd.

Here is what Dom Gueranger has to say on this subject:


  We apply the name of Christmas to the forty days which begin with the Nativity of our Lord, December 25, and end with the Purification of the Blessed Virgin, February 2. It is a period which forms a distinct portion of the Liturgical Year, as distinct, by its own special spirit, from every other, as are Advent, Lent, Easter, or Pentecost. One same Mystery is celebrated and kept in view during the whole forty days. Neither the Feasts of the Saints, which so abound during this Season; nor the time of Septuagesima, with its mournful Purple, which often begins before Christmastide is over, seem able to distract our Holy Mother the Church from the immense joy of which she received the good tidings from the Angels [St Luke ii 10] on that glorious Night for which the world had been longing four thousand years. The Faithful will remember that the Liturgy commemorates this long expectation by the four penitential weeks of Advent.
  
  The custom of celebrating the Solemnity of our Savior's Nativity by a feast or commemoration of forty days’ duration is founded on the holy Gospel itself; for it tells us that the Blessed Virgin Mary, after spending forty days in the contemplation of the Divine Fruit of her glorious Maternity, went to the Temple, there to fulfill, in most perfect humility, the ceremonies which the Law demanded of the daughters of Israel, when they became mothers.


Besides, who has ever known of a joyous liturgical season (Christmas) being shorter than the season of preparation and joyful penance. Please recall that in some regions, Advent is six weeks long, not four like ours. 


  Unlike the Roman Rite, the Ambrosian Rite celebrates six Sundays of Advent (beginning with the Sunday after St. Martin’s Day) with proper epistle and gospel readings. These six Sundays were marked by seven Prefaces altogether: one for each of the first five, with two assigned to the Sixth Sunday of Advent. - Ambrosian Preface Texts for Celebrating Advent


If a table of comparision between the Ordinary Rite and the Extraordinary Rite would interest you, the following may be informative:

Christmas to Candlemas: When is the Real End of the Christmas Season?



The liturgical seasons of the Ordinary Form (OF) and the 1962 Extraordinary Form Calendar (EF)

Although there are several differences between the current General Roman Calendar (OF) and the 1962 Extraordinary Form Calendar (EF), the structure of both of the liturgical seasons is still very similar. Christmastide is not another name for the Christmas season, which extends from Christmas Eve until the Feast of the Baptism of Our Lord in the Ordinary Form of the Mass. Christmastide in the Extraordinary Form (EF) extended from Christmas Eve until the Feast of the Epiphany (January 6). For on this feast three miracles took place: Catholic tradition holds that on this day the Magi come to visit Jesus, that Jesus was baptized in the Jordan and that Jesus performed his first  miracle. Liturgical books of the Mass of Pope St. Pius V call the time following the Epiphany the Time after the Epiphany which is still remained encompassed in the Christmas season of 40 days. 

The 1960 Code of Rubrics defined "Christmastide" as running "from I vespers of Christmas to none of 5th of January inclusive.


  Closely connected with the salvific events of the Epiphany are the mysteries of the Baptism of the Lord and the manifestation of his glory at the marriage feast of Cana.
  
  Christmastide closes with the Baptism of the Lord. Only in recent times has the feast been rehabilitated, and hence has not given rise to any particular displays of popular piety. However, the feast presents an excellent opportunity for the faithful to be reminded of their rebirth as children of God in Baptism. The rite of asperges could be opportunely used at all Masses on this day, and homilies could well concentrate on the symbols associated with Baptism. - Directory on Popular Piety and the Liturgy (119) of the Vatican Website.


Epiphanytide is a whole different subject yet closely related to Christmastide because it follows it.