Zach WeinersmithOffhand thought - I wonder if part of why sci fi and fantasy are so huge now is that a lot of the genres that would've scratched the same escapist itch are now hard to do inoffensively. Like a lot of 19th century adventure literature is oriented around white adventurers visiting strange cultures, romancing hot locals, stealing stuff, and getting away. Setting it among aliens or elves or whatever allows the same story, theoretically minus the dehumanizing.
So in other words, it's not so much that fantasy/sci fi is big as... it swallowed a bunch of other genres that would've once been called adventure, e.g. barbarian stuff, jungle explorers, life in the far north, westerns in general.
A good example to my mind is Flashman. The storycrafting is great, the character is fascinating, but... man, my limit is pretty high for offensiveness in good books, but by book 4 or so I couldn't do it. But, I understand someone redid flashman in the Warhammer setting, because all (well, most) of the dastardly deeds are OK if you do them to fantasy monster creatures.
Yves L. Jardin-Noam@ZachWeinersmith
I don't know. Maybe those genres (sci-fi, fantasy) just absorbed the others? Like market consolidation after a boom of new stuff, the less robust genres were absorbed (it feels icky to use this metaphor, due to the capitalist tones).
Part of my point is that sci-fi and fantasy have equally troublesome histories. Orcs et al as stand-ins for racist depictions are well known, as are eugenics and "rational" supremacy in sci-fi.
Being socially acceptable isn't the case, I think.