Well, the Nightborne are, in fact, differentiated from Night Elves. They have a different idle stance and appear to be quite emaciated and also have markings. I didn't say they were worlds apart, just differentiated. It just seems even stranger to me to have Night Elves and then skinnier Night Elves on the same faction, but then you have Kul'tirans so I don't freakin' know anymore...
I really won't pretend to know the exact reasoning why they don't implement them. I can only use the resources I have to determine a "most likely" reasoning and outcome. The few official statements they've made appear to be little more than PR jargon to "silence the masses". I don't agree with a population excuse in a video game... I mean... c'mon. Cat people exist, but we have to have a viable population to let people play a race? So the most likely reason to me is an inner-bias within Blizzard by someone who has sizeable authority. The likelihood of them not wanting to upset the Horde players even seems less likely to me the more I think about it.