Date: 29 September 2005 WoW Forums Post by Caydiem (CM at the time)
There are still some High Elves in the land, but the majority of those once known as the Quel'dorei no longer fit the bill of "High Elf".
Your comparison with Gnomeregan is a logical conclusion to draw, but incorrect. The Gnomes managed to evacuate a good percentage of their people from their home as it was corrupted from within. Ironforge was close by; they were able to save a good number of their people. Naturally they were indebted to the Dwarves and, by extension, the Alliance, so they fight for their cause.
The High Elves had less of a chance to flee, as they were surrounded by the vast bulk of the Scourge army as they pressed in and destroyed the Sunwell. Still, some survived, certainly enough to show a presence, right?
And so they did after the destruction of Quel'Thalas, fighting for the Alliance, but with the destruction of the Sunwell they found an ever-increasing need, an addiction, for magical power. The remaining Elves were placed under the command of a man named Garithos, who was grossly intolerant of other races and drove the Elves into near-suicidal situations in the hopes they would no longer be his problem.
These remnants of the High Elves -- named Blood Elves in remembrance of their people -- were not stupid, and the commands from their "Alliance" became more and more ludicrous. It was only through the help of the Naga that they were able to survive these grim tasks; time and again, they appeared to aid Prince Kael'thas and his men. The Blood Elves came to realize they could no longer trust in the Alliance. When the offer was extended to join the Naga in service of Illidan Stormrage -- and perhaps find a cure to their addiction to magic -- Kael took it, leaving the Alliance behind.
In short, High Elves are not playable because there are very few true "High Elves" left -- far less so than Gnomes. While there are Blood Elves in much larger supply, they are busy with other pursuits alongside Illidan. You'd certainly not see them fighting side by side with the Alliance anytime soon, not after such betrayal.