I hate to say it, but I do think there is some validity to the argument that faction exclusivity plays a large role in racial fantasy. If they added Night Elf Worgen to the Horde, and they shared the model I use but maybe with some slightly different fur tints, and they had the same class list, I’d feel pretty rotten because overnight my race effectively became neutral. Nobody wants to feel that way, and blood elves certainly shouldn’t as they’ve been playable since WoW’s first ever expansion.
You might disagree with this rationale, but I have little doubt that this is one of the largest reasons why High Elf demands have gone unheeded for over a literal decade.