There are theories about the popularity of horror. My guess is, that it is a combination of factors which include nowadays the notion that horror games are most popular on itch, therefore a new developer tries to release a horror game first. But I think it starts with youtubers driving trafic here. There are probably more views to be made with playing horror game than platformer games.
And regarding the bad uploads here, not everything is being manually reviewed. That is the thing itch does not advertise enough to new users: there is no uploader verification, and no file verification and no human scrutiny to projects. Oh, there is some scans, but that catches only the most blatant stuff. And of course you can be put on the queue for any arbitrary reason, they will not tell you about. In consequence, a malicious uploader can upload bad stuff here, and as long as no one reports it or it does not get caught in any automatic scanning or be put in the queue, it will linger here, gaining the perceived legitimacy of being hosted on itch.
You yourself are proof of this flaw. You thought, that everything here went through some sort of review. The criminals just need to use some sort of shotgun tactics. Upload a hundred games and even if 90 get detected, the other 10 are now on the system and it can take weeks, even years (I kid you not), till they get reported.