Wow. Todd Newton relates an amazingly ignorant comment he observed at COSine 2011, during a panel on Theology Building (in fiction), when an audience member commented:
"Maybe Islam and Buddhism just aren't that interesting."
The question was related to why elements of Christianity appear so regularly in "our" fiction (meaning us here in the U.S.), and what examples are there of other religions making an appearance in SF and fantasy fiction. Apparently the question was met with confused silence, until the above. For the full story, see his post here.
Well, first, I would say that it seems to me that even Christianity may be eclipsed by the Greek/Roman Pantheon. Which I'm surprised no one pointed out.
Unless my fantasy reading is more severely limited than I had thought, usually a polytheistic belief system lurks in the background. But, yes, I have read my share that used Christianity as a building block.
As to why either dominate:
I think it's a simple answer having to do with the audience and the author. The general "we" here is an American/English one (us of the West), of which the majority religion is Christianity in one of its forms and Greek/Roman mythology is prevalent in our literature, media, etc.
It's only natural to write what we know and for audiences to gravitate to the same.
I think it's nothing more simple than that. Unless the author conceives of a cool religion as a major element of the story, then why spend too much time building something that isn't relevant? They should be writing instead about their characters and their struggles.