well, MMario, here's my theory...

In looking for very old carols, one finds that most of them are very religious in nature, because Christmas was a religious holiday. (I mean, it still is, but less so.) So I think those very religious tunes have fallen out of mainstream use. I have been told that even in public places at Christmas time, people have complained that the music was too religious, or complained that it was Christmas music at all and they don't celebrate Christmas. Whatever. (If you don't celebrate Christmas, what are you doing at a Christmas celebration? But that's another story.)

Also some of the songs are rather grim in nature and not jolly at all, and I think most folks nowadays prefer happy-type songs at the holidays.

Just my tuppenceworth. :)