Nah thats not why its popular its popular because the faction war itself is a crappy story device for a MMO.
Here is the problem with the faction war story, it only ever has 3 outcomes, none of which are good because it will either alienate half, or all your player base.
Out come 1: A clear winner and a clear loser, One side ‘wins’ the expansion which ends up pissing off the other side, and half your player base with it. One side ends up either loosing characters or land as a result and no one likes that because no one likes to play teh losing side.
Outcome 2: Pyric victory, one side ‘wins’ but with such great losses that you cant really call it a win, both sides end up losing a lot and characters are either killed off or dont do anything, it leaves neither side satisfied or happy with it.
Outcome 3: The stale mate, the one where nothing actually happens, no one wins, no one loses, it was all just a bunch of balking bakc and forth, no one wins here because nothing actually progressed or did anything worthy of note. Again, no one is happy.
If you look back at the game, the only times the faction war story worked was when it was a back burner story that was just going on in small conflicts in various parts of the world, winter grasp, ashran, the random pvp locations, stone talon. But a actual full blown faction war vs faction war is just pitting both players against each other which will jsut come down to a popularity contest which alwasy results in us putting aside the war to fight a bigger threat.
The faction conflict was a cool ghimic in 2008 when it was fresh, new, and no one was really doing it. In 2024 its kind of stale and lame now.
Just have it where Alliance and horde have a mutual peace treaty but that does not stop various small factiosn within each faction from fighting each other for various reasons.
Backburner side story? fine.
Main A plot or even B plot? Nah always fails.