Alliance is already playing many of its own races, there called Humans, Dwarves, Night Elves.
High Elves are not an Alliance race, and even back before they were made playable for the Horde, they were still not an Alliance race.
Driven to desperate measures, King Anasterian Sunstrider appealed to the human nation of Arathor. The high elves offered to teach select humans how to use magic if the humans would use their new powers to help defend QuelâThalas. Arathorâs monarch, King Thoradin, distrusted magic, but the high elves reminded him that if QuelâThalas fell, the southlands would be the next area to fall before the trolls.
Thoradin reluctantly conceded that the high elves had a point. He agreed to their offer, and soon his armies had joined the war. The united elf and human forces won out over the trolls. History would never see the trolls rise as one nation again. Assured that QuelâThalas was saved from destruction, the elves made a pledge of loyalty and friendship to the nation of Arathor and to the bloodline of its king. As a result of the alliance between Arathor and QuelâThalas, the wizard nation of Dalaran was formed, where humans and elves would study magic for years to come.
The Second War: Alliance and Secession
When the Second War broke out, the high elves had little interest in the conflict, but they felt duty-bound to lend the Alliance of Lordaeron their support. In particular, the commander of the Alliance forces was Lord Anduin Lothar, who was the last descendent of King Thoradin. Lothar's family had aided the high elves in fighting the Troll Wars, and so King Anasterian reluctantly agreed to provide a small number of soldiers and spellcasters who would fight alongside the Alliance.
Then the Horde recruited Amani trolls into its ranks. Shortly thereafter, the Horde burned down the borderlands of QuelâThalas and slaughtered many high elf civilians. Furious at this wanton destruction of life, the elves officially joined the Alliance and committed all their resources to the war.
By the time the Horde was driven back, however, the orcs and trolls had already achieved their true goal: to steal and desecrate many of the Runestones that powered the elvesâ defensive shield. The warlock Gulâdan then used the pilfered stones to power his devious Altars of Storms.
Nevertheless, the Alliance ultimately won the Second War, and most of the vanquished orcs were rounded up and put into internment camps. In the warâs aftermath, the cost of rebuilding was significant, particularly when added to the cost of maintaining the internment camps. Without a common enemy, the human nations began bickering over territorial claims. The high elves began to doubt the value of the Alliance. Humanity seemed to need the high elves, but had little to offer in return, especially now that many Alliance resources went toward maintaining the internment camps.
At last Anasterian rescinded the high elvesâ allegiance to the Alliance. He stated that the humansâ poor leadership had been directly responsible for the burned forests in the borderlands of QuelâThalas. King Terenas Menethil argued that nothing of QuelâThalas would have survived if not for the hundreds of valiant humans who gave their lives to defend it. Despite his attempts at reestablishing diplomatic relations, however, the elves opted to remain independent of the crumbling Alliance. Their departure triggered the additional secession of the Gilneas and Stromgarde nations.
The Third War: Loss of the Sunwell
During the Third War, Prince Arthas Menethil and the Scourge laid waste to QuelâThalas, wiping out most of its population and reducing large tracts of the mighty kingdom to ash in his quest to reach the Sunwell. Yet not all who fell before Arthas stayed dead: the merciless prince raised QuelâThalasâ foremost defender, Ranger-General Sylvanas Windrunner, into undeath against her will to serve the Scourge as a powerful, tormented banshee.
As the undead armies closed in on the Sunwell, a high elf named DarâKhan Drathir aided Arthas by lowering the shields surrounding the Sunwell. In betraying the high elves, DarâKhan hoped to gain the favor of the Lich King. However, the most immediate result of his treachery was an explosion that knocked him unconscious and scattered most of the Sunwellâs powers.
The wizard Borel sensed the mystical energy being unleashed and succeeded in trapping a portion of it inside an avatar disguised as a young human girl, Anveena. Unaware of Borelâs deed, Arthas then used the remaining energies of the Sunwell to reanimate the spirit of KelâThuzad in the form of a nightmarish lich. The Sunwell was left defiled and drained of all its magic.
King Anasterian was slain in battle, and the sole heir to the throne, Prince Kaelâthas Sunstrider, had been pursuing magical studies in Dalaran when the invasion occurred. In the absence of other leadership, Lorâthemar Theron, Sylvanas Windrunnerâs second-in-command, assumed temporary leadership of the high elves.
The few high elves to survive the Scourgeâs invasion quickly grew ill and apathetic. It became clear that they had become addicted to the Sunwellâs arcane energies. Being constantly suffused in magic had fundamentally changed their race. Now that the source of their magic was gone, they were suffering acute pangs of withdrawal.
Prince Kaelâthas returned home and rallied all the survivors he could find: approximately 90% of the surviving high elves. He declared that these survivors would now bear a new nameâthe blood elvesâin honor of their fallen people. The blood elves no longer consider themselves high elves, and they have different priorities and behaviors than their high elf kindred.
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 do not gather in any significant numbers, nor do 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 cultureâonly a past filled with glory and regret.
Like blood elves, high elves can use arcane magic, but most do not because most high elves are not spellcasters. Whether or not they are spellcasters, all high elves are suffering acute pangs of withdrawal in the absence of the Sunwellâs energies. A few high elves here and there have realized the cause of their distress; others have not. A crucial difference between high elves and blood elves is that no high elves have decided to feed their hunger for arcane magic by draining that magic from alternative sources (now that the Sunwell is useless). Even today, though, a high elf might still succumb to that addiction and become one of the blood elves.
A few high elves have chosen to join the Alliance in recent years, but the Alliance as a whole remains somewhat suspicious of high elves to this day. The high elvesâ secession from the Alliance during the Second War left bitter memories. Furthermore, the night elves, who became part of the Alliance during the Third War, saw it as their duty to warn their new allies against the high elves. The Highborneâs part in bringing about the War of the Ancients figured heavily in the night elvesâ retelling of high elf history.
Source: https://wow.gamepedia.com/The_Warcraft_Encyclopedia/High_Elves