Apologies if somebody already asked this, but why are there no dark-type gyms?

Bill: No one’s quite sure, actually. The Association has no rules governing what types gym leaders may specialize in. Dark-type specialists simply … don’t submit applications.

Personally, I’m a little inclined to theorize it has something to do with Karen. Ever since the dark-type was discovered and she claimed the title as the de facto leader of Johto’s Elite Four, she’s gained quite a following, consisting of many dark-type trainers who aspire to be just like her, if not like Grimsley of the Unovan Elite Four. But that’s simply a theory.