Not sure if you know, but high elfs joined the Alliance out of convenience. As soon as the war was over, they promptly left the Alliance, bar a few human lovers.
Also, in response to your comment
"High Elves were once a valued member of the Alliance" here's a snippet for you to read:
However, a few high elves did not become blood elves. When Quel'Thalas left the Alliance, some high elves chose to remain with their allies over their kingdom, and to this day still support the Alliance.[7] Some high elves were exiled from Quel'Thalas after the Third War.[10] Other high elf remnants have chosen to rejoin the Alliance in recent years,
arriving to a cold reception.[2] Modern high elves are a rare sight, and are commonly mistaken for blood elves.[11][12] In consequence, there are so few high elves left on Azeroth today that they cannot be considered a race in anything other than the biological sense. High elves did not gather in any significant numbers, nor did they act as a coordinated whole. They are a very small group of individuals scattered all over the world. As such, they do not have common opinions or goals. Indeed, modern high elves cannot even truly be said to have a culture—only a past filled with glory and regret.[2]
https://wow.gamepedia.com/High_elf
Enjoy the read