Also, the reverse is probably the case, lore wise and from development aspects.
Lore wise, when you start at level one, Borken isles has not yet to be discovered, hell, artas was not yet the Lich King. What we dont think about is that when we level we actually go back in time and start from "vanilla" and continue the trip once again. Being a demon hunter before the events of legion would make no sense since they were all inprisoned.
And also Blizzard would need to add tons of green/blue/epic warglavies every where so new demon hunters would have weapons to fight with. Sounds like a lot of job, but possible.
The reason is probablt lore wise, demon hunters did not exist (inprisoned) prior to the events of Legion.