One reason is December 25 actually makes little sense being the real date according to the scripture account of Luke. In December it is too cold and rains too often in the area of Judea for shepherds to be working in the fields as Luke 2:7-8 states they were when Jesus was born. Also, because of the cold weather the Roman government would not have taken a census in the winter but Luke 2:1-4 clearly states that Mary and Joseph came to Bethlehem to register for a Roman census.
Based on what is stated to have been going on when Jesus was born according to scripture it makes more sense to place the birth of Jesus near early summer. Some might feel that celebrating the wrong day Jesus was born ignores or denies the scriptures are true.
Most religious scholars believe December 25 was chosen to coincide with already established pagan holidays that take place around the winter solstice.