The only reason I can think of, is that they're holding it back or just trolling.
It's nothing to do with the fact the crime organisations use Dark types because they also use Psychic, Poison and Normal types. I doubt it's because it's seen as an "evil" type, because literally every other type, has been used by a gym leader including Ghost and Poison. It can't be because all pokemon that are Dark type are evil, because it's obvious that a pokemon of any species can have a range of personalities (the Houndoom in "Houndoom's Special Delivery") wasn't such a bad guy.
So there isn't really any reason why it wouldn't make sense or GF can't make a Dark gym. I'd assume they just want to keep fans speculating.