An interesting analysis of this question in the above article.
Some of the cited comparisons regarding sales figures by genre are a bit unfair—or at least inappropriate: Horror is much smaller than crime and mystery (which outsells horror by multiples); but crime and mystery comprises a wide, wide swath of the commercial fiction that is published each year.
A more realistic goal would be for horror to catch up with science fiction or fantasy (but probably not both).
As the article notes, Stephen King has dominated horror since the mid-1970s.
We all love Stephen King; but King is nearly 70 years old, and at a certain point, even he is going to slow down. Fifteen years from now, the horror fiction genre will need other authors if it is to survive and thrive.
So now for the big question: Can horror fiction (other than Stephen King novels) make the bestseller lists?
Why not? The article mentions the popularity of The Walking Dead as an instructive example.
Ten years ago, who would have anticipated the mainstream popularity of a television series about the zombocalypse?