I think the problem is that it was the Horde who first got kind of a copy of an Alliance allied race back in TBC when Blood Elves became playable. In vanilla Alliance players could interact with Alliance aligned High Elves and I think a lot of players (both Horde and Alliance) expected High Elves to become playable for the Alliance at some point (and Ogres for the Horde). This may also have been Blizzards original plan until they decided to move the Blood Elves to the Horde to balance the player-base more evenly between Horde and Alliance.
At the time of their introduction the Blood Elves got a sufficiently different theme (vampiric mana junkies) from the Alliance aligned High Elves. One of the High Elves in Allerian Stronghold even emphasizes that she is not a Blood Elf. At the time that seemed to allow for enough differentiation for Alliance aligned High Elves to be added at some point in the future as a playable race so people probably kept hoping.
Of course with the restoration of the Sunwell the mana junkie part of the Blood Elves disappeared but it was replaced in WotLK with the Silver Covenant vs Sunreaver conflict. Once again Blizzard emphasizing in game that there was still a difference (ideological and perhaps political) between Alliance aligned High Elves and Horde aligned Blood Elves.
Blizzard kept this difference alive in MoP, even increasing it with the Purge of Dalaran which saw a direct conflict between the Silver Covenant and the Sunreavers.
Even in Legion Blood, High and Night-Elves were addressed separately in game.
With Blizzard emphasizing the difference between Alliance aligned High Elves and Horde aligned Blood Elves it is no wonder that Alliance aligned High Elves remains one of the most requested playable factions to this day.
It would be unfair to the Horde (players) to make the Silver Covenant a playable Alliance faction in it's current state but I think it also hasn't been fair to the Alliance (players) that one of their oldest allied factions has been copied to/taken over by the Horde, piece by piece. Alliance players can see, smell and hear the "High Elf" experience (figuratively) but they cannot really touch or feel it. You could almost see it as a kind of mental torture
In a way history is now repeating itself with Blizzard "compromise" of Void (infected emo ) Elves. Alliance is getting a kind of copy of a Horde race but thematically changed. Just as with the original mana-vampire theme of the Blood Elves, the "Hearing voices"+Emo theme (with vampire look) of the Void Elves is not everyone's favorite role-playing ideal. I don't think a lot of players come to WoW to to play either a junkie or a mentally unstable character in a fantasy world setting.
Blizzard also repeated the mistake of the Draenei introduction by not seeding the race in the lore/game before introducing them. Alleria doesn't really count as she isn't a Void Elf, she's just a High Elf doing some voidy stuff sometimes.
I think the Void theme can be cool for Alliance playable High Elves but the combination of their emo-theme and tendrils/vampire/goth-looks turns a lot of people off. Perhaps if Blizzard had made Void Elves thematically more like Alleria (proud&strong, having studied to control of the Void rather than being infected with it), ditched the emo-theme and given them a cooler appearance (subjective I know) like a drow (dark skin, white hair) or at least more colorful (e.g. shades of purple/blue) rather than gray appearance they would have been better received. The Alliance seems be getting nothing but gray-skinned allied races.
Nothing will change anymore for BFA but I'm sure Blizzard will continue to try and get Void Elf accepted (with OP racials if needed). Perhaps they will even come up with another "redemption" story like they did with the "original" Blood Elves
