Stephen King and Dean Koontz are both mainstream writers. "mainstream" just means darned near everyone out there reads it, and not just horror fans.
I don't know where you're looking, but there are horror novels everywhere. In order to be mainstream horror, however, they have to sell well enough to be considered part of the mainstream market. Not many horror writers have ever accomplished this. Nor have many romance writers, or western writers, etc. Mystery writers manage it more often than any other genre, but this is because the mystery genre is far larger than any genre except romance.
You're worrying needlessly. Horror is never popular in the mainstream markets. Individual horror novels that are written well enough, and that tell the right story, are popular in the mainstream market, not horror overall. This is true of every genre. The mainstream market simply takes in books and writers that millions of people love.
I hope I'm explaining this clearly. Mainstream is not a genre. It's a market. It's the vast majority of general readers. If enough of them like any novel, in any genre, that novel becomes a mainstream novel.
As a writer, you generally know you achieved this when genre classification vanishes from the cover of your books. When's the last time you saw the words "horror novel" on the cover of a Stephen King novel?
Anyway, this is all meaningless. The only reason to write a book is because you would love to write that book, and would love to read it if someone else write it. If you let something set because you're afraid the market for it is dead, you aren't going to have much success as a writer.