The blood elves are the most popular race in wow, at least amongst the end gamers and legion expec players according to most realm data sites. Their alliance counterprat the high elves have been coveted since the game was introduced in 2004.
With legion introducing the new fancy night elven highborne race - the nightborne that seemed to be liked by all elven lovers alike and coveted by the blood elves ('ve never seen them show interest in night elf related stuff till now ) but since they are interested, I wonder how horde players would feel about high elves being available on the alliance side if it would mean nightborne available on the horde side.
Similarly, alliance players, seeing a potential awesome part of the night elf group being introduced in the night elf empire minded nightborne - bringing high civilization and arcane magic aspect of the night elves back into the picture - the nightborne provided a purer night based elven group, deepening and diversifying the night elf group well beyond the tree hugger tag it gained during WoW. They would be quite the boost to the night side of the elves especially with the potential for a reuinted night elvendom and all the wonder that could entail - would you as an alliance player be willing to have this cooler version of night elves available on the horde if it meant you could get playable high elves.
This is what you get
High Elves - the games most attractive race currently now made playable on the alliance
Nightborne - a really cool new version of night elven highborne as seen in Suramar now available on the horde.
It goes without saying that blood elves will continue on the horde, and nightborne version of night elves also available on the alliance.