I love how open ended this question is. It leaves plenty of room for insanely wild conspiracy theories based on the essence of Pokémon and humor. You could go with ideas that the regions are all based off real life locations and go from there (a way to lead to misconception). You could go with the idea that Dark type Pokémon were originally were never intended to exist, but I truly deeply believe and should preach that the main reason that there is no dark type gyms in existence is because there are no bathrooms in the Pokémon games at all. Just think, oh this is all real, there are wild animals everywhere I am trying to catch but in reality they are not real. Why? Because the trainers are either in the realm of science fiction or the Pokémon are evolved forms of humans who can go potty w/o shame (Darumaka fro example)... Oh but you will say there are crossovers between Pokémon and games with bathrooms like say the Bag of Dragonite in Earthbound (which if you could put in real context "Dragonite" is probably just a slang word or derived from some strange language) well here's what I say to that...yjhkgdsakfjg I say nothing because crossovers do not explain canon.
Or all the good Dark types have a super huge weakness to Bug types which are clearly better in every stat.