And what about all those High Elves who live scattered across Azeroth? What about the Quel'dorei lodges as Quel'Danil or Farstrider (and previously Quel'Lithien)? And those who live isolated supposedly in Greenwood? Or what about the High Elves who are settled in Outland or other planets?
All of they are High Elves and remain loyal to the Alliance, yet have nothing to do with Dalaran and the Kirin Tor. Actually all these High Elves have no leader or faction to represent them, and yet there they are, fighting for their ideals along with the other races of the Alliance.
In fact Blizzard already tried to lump all Quel'dorei loyal to the Alliance in one faction (
Silvermoon Remnant) and although that faction did not reach to be implemented, that does not mean that the Silver Covenant is the only organized group of High Elves fighting by the Alliance: that would be a cruel lie.
While it is true that so far the Silver Covenant has been the only group of High Elves who has taken an active part in the Alliance, that does not mean that all High Elves are congregated in that faction. There are many more Quel'dorei who fight every day for the Alliance and are not members of the Silver Covenant.
Now, considering the atrocious hate which has Blizzard towards the Quel'dorei, it is not surprising than for Blizzard the Silver Covenant is the only remnant of the High Elves in the game, but we know very well that this is not so, whatever Blizzard says.
In short, there are lots of High Elves of the Alliance who are not represented by the Silver Covenant or by Vereesa, and certainly the appearance of Alleria would cause that all of them gather around her forming a single faction: together they will much stronger. However this does not mean that, for reasons of gameplay, a small and fragmented faction of High Elves (such as the Silver Covenant) act as representative of all the Quel'dorei people.