I'm reviving this thread because it's really interesting to me. My work is very theological. I'm involved with a group called the Anselm Society, an artist's guild with the mission to "revive the Christian imagination," so in some ways I'm on the front lines.
Perhaps one of the problems is that some of the greatest writers of Christian fiction in recent memory, the Inklings, had such a large impact on modern faith (in particular Christian evangelical faith) and on writing itself, that to even delve into certain topics means to tread where the audience has (or thought they have) gone before. To have any impact, it must be a foundational, wide reaching impact. It takes something huge to make religious people think differently about anything (think works like Dante's Comedy or Milton's "Paradise Lost," both of which introduced radically new ways of thinking about theology while simultaneously resting on the shoulders of the giants that came before them).
I'm struggling a bit to convey what I'm really thinking here. This is partially a temperamental problem. Most artists are temperamentally liberal (not liberal in the political sense, though there is significant correlation). Most people on here I would guess are on the open end of the "openness to experience" personality trait expressed in the Big Five personality theory.
Most religious people however are temperamentally conservative. They don't play with new ideas; they preserve ideas that are already, in their experience, tried and true (which is a culturally valuable thing; I'm not trying to badmouth anyone). That makes it really difficult to introduce new things. If it isn't like CS Lewis or Tolkien, or if it doesn't measure up to some kind of other (sometimes quite arbitrary) standard, there is no way a bit of writing is going to be successful even if it's right. That's simply the temperament of the audience we're trying to write to.
It's no wonder then that there are few Christian artists (or ones that are recognized in the mainstream whose work is explicitly Christian). To be successful *and* unique, a Christian artist can't market to Christians (at least evangelicals).