So Blizz is ok stretching the lore enough to retcon draeneis into protoss space sex idols, but the idea of a non-elven demon hunter? why that's crazy talk! we need more elf classes to help with their low pop issues!
The demon hunters also have a shitload of modelling work, it's not just lore reasons that they're elves only.
and "why"? Cause there were aspects of garrisons that were fun and aspects that weren't, and people have been asking for more class specific quests/content since the game came out. Speaking as someone that has always had one of these spaces as a DK ,they're fucking awesome. I chat with random death knights in Acherus all the time.