Because of the church and the need to establish a state religion.
It's a lot easier to change the name of a festival than to stop people celebrating. Look at the trouble the Puritans had during the Commonwealth.
'More mischief is that time committed than in all the year besides ... What dicing and carding, what eating and drinking, what banqueting and feasting is then used ... to the great dishonour of God and the impoverishing of the realm.'
So wrote the strict protestant, Philip Stubbes, in the late 16th century, expressing the Puritan view that Christmas was a dangerous excuse for excessive drinking, eating, gambling and generally bad behaviour.
This view was made law in 1644, when an Act of Parliament banned Christmas celebrations. Viewed by the Puritans as superfluous, not to mention threatening, to core Christian beliefs, all activities to do with Christmas, both domestic and religious, including attending church, were forbidden. The ban, however, was unpopular and many people continued to celebrate privately, albeit in a far more restrained manner than in Elizabethan times.
A more openly festive, if slightly subdued, spirit returned following the restoration of the Monarchy in 1660. Old customs were revived, and Christmas as both a religious and social festival was celebrated throughout society.