Why is this horror Genre?

It wasn't a horror in my opinion, but I imagine it was labelled as such because it was an adaptation of an Edgar Allen Poe short story. Also, the horror genre garners more attention than most other genres - I watched it in hope of watching a horror. I'd say it's more of a "mystery" or a "thriller". It's a Brad Anderson film, and a good one at that. I rather enjoyed it.

Maybe you are thinking of slasher films like Scream & torture porn like the Saw films where the "horror" is graphic violence and sudden shock editing. They make you jump in surprise and wince at the sadistic violence being inflicted. However, they have little psychological or thematic depth or emotional plausibility-as opposed to realism-few films are realistic. I would put this as a horror film due to the morbid and grim setting and melodramatic plot turns. It also has a theme of madness and what we consider sane or not and how we treat it.