Except the faction split is close to equal, according to the (unreliable) information available to everyone who doesn’t work at Blizzard. (While it’s unreliable, it’s still the best we’ve got available to us as a source.)
The difference is the hunger for PvP. Faction split doesn’t equal desire to queue for BGs, as you can clearly see based on 3rd party data.
It’s not like everyone online is always going to queue for BGs. So the horde simply have more PvPers as a whole in EU, thus resulting in queue times for hordes but not for alliance.
What’s interesting though, is that Alliance have queue times for WSG, while horde rarely does. So perhaps many of the (true) Alliance PvPers moved to WSG instead of AV?
Btw a better solution is to just bring back realm-only queues. That way, nobody is ever “guaranteed” a queue pop at all hours of the day, and would instead be very different from server to server. Would also serve as a motivation for people to balance out realm faction splits more evenly as well. (Because the realm faction splits are very volatile right now, and they aren’t punished for it because of xrealm matchmaking. So it never motivated the PvPers to balance it out themselves before the honor system started in phase 2. They had already confirmed xrealm matchmaking in PvP early on in 2019 after all.)
Realm-only matchmaking would basically make it this way for everybody, because you’d develop rivalries and so on and want to kill some more than others etc. and you’d recognize people, which you barely do now. (If at all.)
More on this effect in: