My observations make me think that there are just a relatively high number of indie horror games compared to other genres of indies. I reckon it’s because they’re probably easier to make - you can focus on just building a world and putting in a few scary things rather than worrying about gameplay and complex mechanics if you’re not interested in spending your time designing and developing those things. Also I imagine there’s an audience for people who like horror who otherwise don’t care that much about video games.