I read a book not too long ago, Drawing on the Power of Resonance in Writing written by David Farland, a novelist who had done extensive market research and focus testing in the film industry. The industry has spent billions (trillions?) of dollars trying to figure out what sorts of story appealed to what sort of audiences, and while you cannot, of course, make generalizations that apply to everyone of a given demographic, that's not the game. Producers want to cast as wide a net as possible to garner the largest possible audiences. The outliers are irrelevant unless they number high enough to justify trying to market to.
Anyway, one of the biggest takeaways in the book was the generalization of what "kind" of stories appeal to the broad masses at different life stages, based on emotional draw. SFF both provide readers with a sense of wonder as a primary emotional draw, and "wonder" appeals most strongly to younger demographics - until the teen years or so.
I am not saying that SFF is juvenile, and I am not saying that SFF stories do not contain other emotional resonances, but in the eyes of the general public that sense of wonder, of the new, of the strange and unusual is what unifies and typifies SFF. This may be why many people look down on the genre, or at least don't take it as seriously as a form of literature.
Of course, we know better, but that understanding does not change the general perception of the genre. When asked what I write I usually answer with a pithy "Whatever they pay me to write," or a more serious "alternative historical mysteries and thrillers" not because I am ashamed of writing steampunk, but because I don't want to have to explain it.