@Hanna_Brassheart got it right on.
The first term for Christmas was X-mas. It isn't bad, but people in this day and age are using the X, when they don't even know it's good, to make the word Christmas look good for "everyone". And that is the reason most Christians don't use it so they don't make the X-mas people feel like they are accomplishing something.
To go even farther, when people say Happy Holidays instead of Merry Christmas, they are still basically saying Merry Christmas. The word Holiday originates from the two words, Holy Days. One of the Holy Days is Christmas. So like I said, the anti-Christmas people are still losing.