I'd say horror is about 1 or 2 and likely to stay there. And it was there when Stephen King was at his horror peak too. Stephen King is Stephen King - he's an extraordinary storyteller with an equally extraordinary work ethic. Regardless of the genre he chose, he was probably going to be wildly successful. He wrote horror stories because they were the stories that came to him, and some of them become movies that were 'scary' more than pure 'horror', and thus he entered the mainstream in a massive way. This is a wildly generalised summary of a long, rich career.
My own theory about why horror isn't more popular is this: chicks don't dig it. And when chicks* don't dig a book genre - either to read themselves or to buy as a gift - it's very, very hard to get it consistently on any bestseller lists. Most women I know - including me - would not voluntarily read a horror novel or watch a horror film. Alien is probably as horrific as they'll go, and even then it's a 'space film' so we can convince ourselves that it could never happen on earth, ergo, it's not as horrific as it could be. We'll watch/read crime stories that come close to being horror, but we're mainly not interested in horrific - really nightmare-creating - stories. The women I know who read voraciously (and some of the men) will not go anywhere near a horror story. Of course there are exceptions to this. There are lots of women who've read Stephen King. But he's Stephen King. He's a genre unto himself.
The comments section is there for anyone who wants to seriously go to town on my theory.
*Culturally ironic use only.