Why do you think horror is a good fit for this film?

Because it features a good amount of gore and the tone of the film is in keeping with traditional horror. Why would you think it's NOT a Horror film? It's listed as Horror everywhere else, including IMDB and Wikipedia.

I ask because horror was previously removed from the page and there's already 3 genres added.

Yeah. I think someone is going around and removing Horror as a genre tag. I've noticed it happening with a few others on here that are traditionally described as horror but are not just horror. My guess would be gatekeeping.

This is one example.

https://www.themoviedb.org/movie/7191-cloverfield/discuss/5ee86d569715ae0021a7d3c1

Someone removed Horror as the genre tag on Cloverfield despite the fact that every other source calls it horror and one of the other mods keeps closing the thread whenever anyone complains.

While American Psycho has some elements of horror (and certain parts of the movie fit that description), it's not really a horror movie like what we expect from the genre. Interestingly, while checking around for some old reviews from when the movie was released, the critics that thought it was a horror film very often also added comedy as a genre (which is a big stretch as well, since it's very subjective). I think the current genres are fine.