This is all that needs to be said, really. There was never going to be a single new elf race that was available to only one faction. The outrage would have been EPIC.
The only two options were either to have Nightborne be neutral and go through a quest chain where your NB chooses a faction or to have two new elf allied races one for each faction. I'd have prefered the first approach personally but for some reason Blizzard didn't like that so... here we are.