First of all there’s more of King to adapt compared to Austen’s six complete novels. Second of all it’s not just the straight adaptations of Austen but stuff like Clueless, Bridget Jones’s Diary and ‘Bride and Prejudice’ and something currently in rpoduction called “Jane Austen Handheld”. Not to mention literary take offs like the recent “Pride and Prejudice and Zombies”. I would say that in the last 15 years more people have seen stuff based on her work than King’s. There is a veritable Jane Austen industry with tons of supplementary material on prominent display in bookstores.
Understand that some of that stuff, especially P&P&Z is actually making fun of Austen. And volume of available material really is immaterial, this is the cinema world that has twice released 2 movies based on the same book nearly simultaneously (Haunting of Hillhouse by Shirley Jackson). And really if you want to talk about a veritable industry with tons of supplementary material that IS King. The guy is the Kiss of authors, he’ll greenlight any thing. An Amazon search on Dark Tower comes up with 8000 items and almost all of them actually are related in some way to King’s series.