It doesn't have to, but Blizz is too lazy to try to introduce anything new. Hence, DH only had 2 specs. New races are just lazy reskins with short stories.
Heck, it is actually rather amusing that Blizz advertised BfA as having all these amazing new races which, in addition to just being lazy reskins, aren't actually a feature of the expansion; they are reputation rewards only like pets and mounts.