Adult - You don't need to ask, do you?
Slice of life - Easy to make thing relatable, particularly given that furries are often stand ins for people, and lets you explore concepts and give people good analogs. Exploring sexuality is a very common theme for this reason... and makes for easy porn. But also concepts like fitting in, or dealing with school, and so on.
Fantasy - Fantasy, by virtue of access and exposure, is just more popular than sci fi. If the original DnD was sci fi, that might be different. But also people play WoW, Skyrim, and so on. I looked at some top 10 game lists, and of the most common top 10, 5 were fantasy. Only 1 was distinctly sci-fi.
Comedy - Most people don't unwind watching a brutal murder, comedy is far more appealing. But also, everything else done wrong winds up as comedy.
Action & Horror stuff is hard to do well. There's lots of things, mind you, that might be horrifying, but the effort to write good horror is high, and most people would rather something comedic. Now, good comedy is probably just as hard to write as good horror, but bad comedy can still be funny, while bad horror becomes comedy. As far as action goes - that tends to drift towards comedy anyhow, particularly when trying to communicate it in stills. Or you end up making it super edgelord... and we're back at comedy.
Sci-fi - the problem with sci-fi is, unlike fantasy, it's harder to cleanly communicate. Fantasy you toss some armor on guys and remove TV screens and there you are. We have a pretty well defined visual language for fantasy. Sci Fi, on the other hand, takes a lot of world building and background details to make work. And even if you do do it, it can look a lot like modern unless you make it cheesy or over the top.