Because the Christian church primarily based at the time in the pagan centers of Rome and Alexandria pressured by the Emperor Constantine, beginning w/ supplanting the 7th day Sabbath w/ 1st day 'Sun'day sacredness, adopted all sorts of pagan holidays attempting to 'Christianize' them in order to artificially grow the church and Constantine's political power.
So called "Christ-mass" pasted over the centuries old traditional date celebrating the 'birthday' of numerous pagan gods was just one of them.

In the pagan practice an evergreen tree was an *altar* under which *gifts* were placed in an appeal for blessings from the pagan god.

Here is an excellent account of the history of pagan catholicism

https://youtu.be/PzYQcUVBt...