Like I said, out of Spite and Schadenfreude because you can't psychologically cope with the Alliance getting something. The High Elves were Part of the Alliance since WoW Classic, far longer than the Blood Elves were a part of the Horde. We hat the Farstrider Longes back in Classic as well as Alliance allied High Elves in Locations like Theramore but also In Stormwind, as far as I remember, we had Alliance allied High Elves back in BC, we had them in one of the most prominent roles of all Alliance Races back in Wotlk, we had them in Minor roles but still there in Cataclysm leveling as well as in Zul'aman, we had them in a prominent role in Mist of Pandaria, they were absent back in WoD but had yet again a prominent role in the Hunter Class Order now in Legion.
They are besides the Humans the Alliance Race with the second most Screentime. Not making them playable is nothing short of trolling and bullying the Alliance Players. Its like if you would make the Orcs or Forsaken unplayable. Considering their impact in the Lore, the High Elves are much more of a force in the Alliance than minor Races without any prominent Roles in the Alliance like Draenei, Night Elves or Gnomes.