Something important - but which isn't mentioned here - is that the Oscars, being what they are, consider it a duty to nominate films with progressive (and often relevant) messaging.
I love film. I watch it. I dream about it. I make it. And while I enjoy the occasional horror film, I'm forced to admit that many of them do not pursue or promote a higher cause. For this reason, we were taught in film school to shy away from the genre. Horrors too often rely heavily on the most basic forms of story-telling. After all, one reason they're cheap to make is because they're easy to make. And, true to form, the worst in show (bar none) during graduate thesis film showings were horror.
Many film professionals, including me, strive every day to produce work intended to entertain, educate, and inspire. The goal is to communicate a message or series of messages we consider not only inspiring, but valuable. Six people being torn to pieces by an ominous force isn't necessarily valuable (though it can be fun as hell).
Action movies are another example of what I'm referring to. Look at Robert Rodriguez. If you haven't seen El Mariachi, do your best to sit through it. It's a god-awful film, but it propelled him into the spotlight and eventually his career because he did it for (allegedly) $7000 (Though I've read some pretty compelling arguments that it's closer to $100k, especially after adjusting for inflation). And it does what it's designed to do, and no more - Gets your juices flowing, wows you a bit, etc.
If you guys haven't seen Moonlight, I recommend it. That's an Oscar-worthy film. The Witch?......maybe not.