I'm not sure this is true or even have any evidence to back it up, and I certainly mean no disrespect to anyone that may love fantasy, or even writes fantasy, but to me, fantasy just seems easier to justify. You don't have to explain why vampires or magic exists, the way you might have to explain how a technology exists or how humanity evolved to the point of space travel. I personal think it's much more easy to simply say, magic or supernatural creatures are here and common place.
I'm not a big fan of fantasy and perhaps that's part of the reason I feel this way, but I love True Blood and Game of Thrones, both of which are steeped in fantasy elements. And in those cases, I'm more ready to overlook things...or suspend my disbelief towards things I know don't exist aren't real because they take no explanation in order to create them and in both cases neither go into detail to explain how their fantasy elements came to be. It's simply a world where those things are real. And that's fine.
On the other hand though, science fictions is largely built on real world elements...it has to be by definition contain some or at least be built upon some sort of science where as fantasy is anything you want it to be. So because of this, fantasy becomes more easily accessible to the masses because it requires less work to get into it and consume. Vampires don't require the same level of critical thinking space exploration might warrant.
And I'm by no means trying to level a degree of intellectual superiority against one genre or the other, or the people that prefer one genre over another. As I said, this is merely my own opinion here and I don't really have anything concrete to back it up with other than my own interest in sci-fi and fantasy.
On the other hand, it could just be the current trend. Zombies are incredibly popular right now as well, but like most trends, it seems to have peaked or be near the peak.