An infamous example of what happens when a prestigious mainstream author writes a horror novel is Brett Easton Ellis's American Psycho. A genre book, by contrast, is not likely to get any mainstream reviews, and thus has less chance of getting into the hallowed halls of literature than a hulking hairy monster has of making it through the eye of a needle. One way of breaking this down is to use the form vs. content distinction.
Realism is also in the nature of the beast.
I think of these as the King vs. Lovecraft parameters. In the real world, things happen. In the real world, people have emotions. Many mainstream authors, by contrast, seem to go out of their way to avoid storytelling, in order to differentiate their work from the literature of the past. The characters in modern novels are flatter, more abstract.