They can because it's their story. Regardless, High Elves who rejected the new path for their people still exist and the npc's serve their purpose by reflecting it in game. However, the blood elves and void elves are the legacy and continuation of the Quel'dorie. This is why I believe that inevitably the remaining High Elves will die out while the blood/void elves live on.
The problem with giving High Elves to alliance players is that it regresses the narrative of Quel'thalas. Blood Elves were High Elves. The attack of the scourge decimated their way of life and were forced to find a new path for survival. The Horde, through Sylvanas, was their Salvation. Sure you had your special snowflakes like vereesa who rejected the new path but they were an extreme minority from a race that already lost a majority of its population.\
The story of the Quel'dorie needs to continue to evolve. Golden Eyes for Belfs and Void Elves were the next step. Playable High Elves would spit in the face of progress from a lore point of view.