My theory as to why these races didn't get any hair is because they already did. 8 unique hairstyles for each (with Viera getting alternative slightly different ones). And since 5.0 they released 8 hairstyles for everyone else.
So now that we're "even", there is some possibility that they might get more hairstyles, either in a batch or along everyone else, starting with 6.0.


We know that they are very capable to release a new race in a short amount of time, if the priority is set accordingly. You could argue that there wasn't much time to make things work for Viera but Hrothgar was of "their own making"... and they have more limitations, though most likely because the priorities were set like this. It really isn't rocket science to make a hair work for another race, but they can probably only whip out so many of them at a time.
8 / expansion? It were 7 between 4.0 - 5.0 and 9 between 3.0 and 4.0.