TNR has a piece by one of the writers for Buffy and the Vampire Slayer (and an assorted other sci-fi shows) about the appeal of some stories (e.g., Harry Potter) and the lack of others. Her basic thesis is that the story needs a "Chosen One" central spoke to anchor the axis of the narrative. That sounds fine, though I have to wonder why David Lynch's Dune tanked so much (just a bad film?). I don't have any grand theory for why sci-fi and science fiction are ghettoized in the norm but occasionally break out to become cultural phenomena. It seems like one of those stochastic "Tipping Point" dynamics which you know will happen, but don't know when. I will offer that in literary science fiction the boundary is pretty clear: any science fiction work which has widespread acclaim in the broader society isn't "really" science fiction. I've talked to people who have read Brave New World and Slaughter House Five and simply not understood what I was saying when I referred to those novels as science fiction. My high school English teacher told me that Brave New World was more a "dystopian classic." Yeah.