Well, when I think about it, the reason why you dont see it much at sci-fi/fantasy ones is that, as far as I can tell, it often just seems to be alot more expensive.
Think about it: Anime characters are, well, animations, so no matter how cool they look they're still flat, 2D characters, and their outfits can only have so much detail to them, otherwise they become nigh-impossible to draw (and they lose that proper anime "look" if they're too detailed).
But characters out of sci-fi and fantasy TV/movie series though are basing their outfits often on those used by actual actors, which are *way* more complex in terms of detail. And any cosplayer wants to look as accurate as possible regardless of if they're the ones making the costume or if they're having someone else make it for them.
I can say from experience that even relatively "simple" costumes cost a lot of money. Seriously they're freaking expensive.
And just going to a convention at all is notoriously pricey. The blasted hotels know full well they can jack up the room prices and get away with it, so they ALL do that. I remember a lot of my early convention experiences involved rooming up with like 4 or 5 friends in the same blasted room, because it was costly and everyone needed to pool their money together. These days I can just go entirely by myself without worrying about that nonsense, but for a great many con-goers, they crowd together like that just because of how horribly expensive it is to even go.
So yeah, that's my theory. High detail in certain specific types of costumes takes an already very expensive hobby and makes it cost even more.