I don't know if it's because the horror genre in gaming isn't as old as many other genres or because things get muddled when horror can be both a theme and a genre, but I think horror games have always been lacking in diversity. I mean, you can make just about any kind of game horror-themed, be it a platformer, RPG, first-person shooter, you name it, but those games wouldn't necessarily be considered a part of the horror genre. When talking about games that are within the genre itself, there were barely even any 2D horror games prior to indies. And even then, a lot of horror indies have started with the mentality "I want to do a pixel Silent Hill/Resident Evil/etc." I can't really blame developers for copycatting when experimentation isn't always welcomed. There are still people who are mad that Resident Evil 4 didn't play like the prior Resident Evil games, not to mention all the hatred out there for 5 and 6.