A question, if you will:
How do you reconcile the existence of the Nightborne with the Quel'dorei history?
I mean, lets be honest here. Before Legion, they didn't even exist. Nobody had ever heard of an entire city of lost former Highborne that somehow magically survived the sundering (despite being literally within spitting distance of the Well when it exploded), the cataclysm, and everything else. No mention of them existed in any prior lore. And then suddenly Poof, entire lost city of yet more elves, and a gigantic massive retconning of everything we know about the first war and the sundering.
Now, why do I mention Quel'dorei history? Because the Nightborne have been in contact with the Quel'dorei. They KNOW how the Night Elves reacted after the fall of Ashzara. And you expect them to be "thankful" for the fact that the Night Elves "saved" them from the legion while they were trapped in their long, self imposed exile? Don't be daft. You and I (and they) all know that if it wasn't for that bubble around the city, the entire lot of them would currently BE Blood Elves because they would have been banished across the sea 1200 years ago with the rest of the Highborne.
- - - Updated - - -
True, but that's because classic "Night Elf" society was built on an INSANELY rigid caste system. You were either Highborne (arcane magical, graceful, aristocratic, arrogant) or you were "Night Elf" (savage, aggressive, forest dwelling, nature magical, still arrogant).
Thing is, the "playable" Night Elf faction basically NEVER included the Highborne. The "savage" version was your entire exposure to them in WC3, with the exception of the Naga, where you are told "This is what happened to the Highborne who followed Ashzara". And then your basic introduction to them in WoW lore was "Oh yeah, we BANISHED the entire lot of them who survived the Sundering across the sea where they basically became an ENTIRELY DIFFERENT FACTION known as High Elves. Whoops.
So yes, the reason you can't play your Aristocratic, Arcane Magical Night Elf Highborne is because other than the Shendralar exiles, they basically don't actually EXIST any more in Night Elf society: the Night Elves essentially eradicated them. So you can blame the Night Elves for that.