Christmas music is special for quite a few reasons, none of which have anything to do with the holiday.
First, a Christmas tune that gets popular will outlive the rest of the artists library. The only time you hear any Bing Crosby these days is at Christmas. The Waitresses? Same thing. If it wasn't for their holiday tunes they wouldn't get airplay anymore.
The other thing about Christmas tunes is that it is often the only time the average person hears "traditional instruments". Trumpets, flutes, oboes, etc. aren't part of popular music anymore but are still heard in many Christmas songs.
The holidays are the only time of year we don't disregard older music.