The Witches of Dathomir were founded in 600 BBY. TOR takes place 3 millenia before that (~3600 BBY). Including them would be completely out of the timeline for them.
This has been a continual issue when people ask for certain EU groups to be included in game: TOR takes place a *long* time before the events of SWG or the SW movies and shows. Unless an organization is incredibly old, it's not likely to have been founded before the game's timeline.
There's a practical reason not to include them as well. We wouldn't have to listen to players demanding gigantic Rancor mounts for their Dathomiri.