I remember hearing this from a Youtuber that DH cannot get a new spec or really get a revamp because the rigging and coding it uses essentially breaks the game and requires too much manual adjust. Essentially DH models and rigs act as separate races because of how unique the movements are. They would need to rewrite and modernize the whole class code to fix the core issues or to do things like add new specs, races, ect.
Its possible that DH may never see any extensive changes as the class just isnt worth the time investment.