Christendom, like other religions before it, assimilated former religions to forge its own traditions. This is very much the rule rather than the exception when a new religion begins to dominate an older one. It is easier to get people to come to your point of view if do not change things too much.
There are a number of pagan traditions that were assimilated into the Christmas tradition, and not all of them were done simultaneously. For example, the Yule log is much more recent than celebrating Christmas on 25 December (and that date is not universal, by the way). Let us look as some of our customs that are not Christian at all.