Why do you think there haven’t been any Gym Leaders who specialize in the Dark-type, despite there being multiple Elite Four Dark-type specialists?
Difficult to say. In Gold and Silver it was likely because there were so few Dark Pokémon and all of them except for Umbreon were very late-game, but that excuse is really gone by the third generation. I suppose you could suggest that, because Dark is the ‘evil’ type and Dark techniques focus on, basically, fighting dirty, while Gym Leaders are supposed to be educators and pillars of the community, they just don’t want Gym Leaders who focus on Dark-types; they want trainers to have more experience before they start playing around with that stuff. But that’s just me making stuff up; I don’t have any particular support for that interpretation.
Pokémaniacal 