Just my speculation and not really based on facts:
I suspect the "secular" artists aren't doing as many Christmas songs because there are so many new ones done by Christian artists.
but that means that if you don't listen to the "Contemporary Christian Music" (CCM) genre then you won't hear the new Christmas songs, which is sad, because there are many new ones since the '80's and a lot of them are extremely well made.
a lot of times CCM will spill over into the "secular" music and will be played on the radio and other media distribution along with non-CCM music (there are several examples I could state and you might have heard some of the more popular ones that reached non-CCM charts even tho the songs themselves are actually CCM songs). However, I can't think of too many Christmas songs which have bridged that gap.
MAYBE this one did...I'd bet it got airplay on non-CCM stations, but I don't know specifically how well it did.
http://www.youtube.com/watch?v=MpkI7GW2V34 (careful, tho, it's s tear-jerker!

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