It's because they don't have too many restrictions on itch, just that it is labeled NSFW for people like me who don't want to see it. But for the people who do, they know where to go for it (itch), which brings more developers here. Also to comment on some of the other posts in this thread, you don't have to make NSFW games as your first indie game. You're not competing with big games, as most of the people who will be buying your game won't be playing AAA anyways.