This is likely because Dark, Ghost, and Bug type special attackers are less common.
The only special dark type move that is viable and not exclusive in singles is Dark Pulse. Physically, you can use Sucker Punch, Night Slash, Knock Off, Crunch, etc...
As for ghost types, shadow ball is the only viable special attacking option, although Hex could be considered as well. However ghost has got Poltergeist, Shadow Claw, Shadow Sneak, and even Phantom Force. Bug types have Quiver Dance and Bug Buzz however U-Turn is the most common bug-type move. Bug types aren't really good in general.