It was their decision not to do that, for whatever reason, perhaps they had a story idea in mind for the High elves, or maybe they didn’t feel the High elves as a group would ever accept becoming Void elves, we don’t know. You can’t turn it around and say “Oh well we have some alliance npcs, so they belong to us!” That’s not how gameplay balancing works, as you do not play as those characters.
You can’t claim a race that isn’t playable, especially when that race already exists on the other faction. When the Blood elves were first introduced Metzen tells all the players “Blood elves ARE our High elves.” and that was the official stance Blizzard took.
There’s a difference between claiming the identity of a race that has been playable on a faction for over a decade, and trying to claim the identity of a race that has never been playable. High elves do not belong to the Alliance, so you really have no place to tell anyone whether or not they should have access to them.