Why do you think there hasn’t been a Dark-type gym yet?

Difficult to be sure, but I suspect that it’s because Dark is the “evil” type (literally, in Japanese) – their strongest and most distinctive powers are all pretty much the antithesis of good sportsmanship, relying on trickery, subversion and malice.  Although I don’t think any characters in the games have ever actually said it, that’s a pretty good reason not to have a Dark-type specialist be front-and-centre in your sporting organisation’s public relations, education and outreach programs (and just look at the Dark-type Kahuna in Sun and Moon… the results sorta speak for themselves).  Of course, anyone who actually trains Dark Pokémon knows that they aren’t really evil, just pragmatic, and trainers who are already skilled can do wonderful things with them, but, well, their optics just aren’t good, put it that way.  You also probably don’t want to encourage new trainers to pick them as a speciality.