True, Dark appears to be under-represented. I like to think of Dark-type trainers as chaotic and consequently lacking the motivation to build and maintain a gym, kinda related to the protagonist in a way. Maybe they're independent trainers who had their own journeys and were asked to join the Elite Four. Then again I'm just roleplaying imagination here.
Maybe Game Freak screwed up by having valid Dark gym ideas in Ghost type ones, using them up.