I’m a theistic science fiction witer, or to be more accurate I’m a theist who writes science fiction, among other things. There seems to be nothing intrinsically atheistic or theistic about science fiction that would lead me to see why one group would be more attracted to it over the other, if, indeed, that is the case, I don’t know. I suppose if I were to have to make a choice I would say that the allure of “science” or the air of “science,” what Ken Wilber calls “narrow science,” is appealing to atheists because it serves as an alternative to God and gives them a sense of authority. “Science” (hardcore materialism) has become the sort of gatekeeper of all that is real in the modern world, for various complicated reasons too many to go in to here. Without a belief in God (used as broadly or narrowly as you choose) one still has to believe in something, and that something might as well be “science” as the new world authority.
Moving on to the fiction part, well, humans are remarkable storytellers. Everything is a story, built up around facts. Facts in themselves are just pieces floating around waiting to be attached to a story that explains how they all fit together. For an atheist instead of writing about God why not write about the new authority, “science?”
Of course all this is speculation; the ramblings of a man with too much to say and too small an audience.