How do you not know this? Alliance has been losing popularity hard since Cata happened, which made them face defeat after defeat in the questing zones and since then the Alliance leaders have been mostly incompetent. Racials being way more favorable for the Horde for a very long time caused a snowball effect where top guilds moved Horde so that they could get world first easier, which made more and more guilds move because that’s where the raiding population was and the dedicated pvp.
It also doesn’t help that there have been numerous instances where Blizzard poked fun at Alliance at Blizzcon, the biggest example still being allowing that one guy to go on stage and call the entire Alliance a buncha f-words and etc etc, cussing the faction and those that play it out. Questing has remained more favorable for the Horde, BFA’s biggest example being that all the Uldir bosses were introduced in the Horde questlines but Alliance didn’t even have any idea why they were going into that raid except for a brief mention of it at the end of a dungeon that Brann was gonna go check it out.
Ah right, as for the Night Elven players of the Alliance they got to enjoy cinematics about how Saurfang felt about the whole burning of Teldrassil, instead of cinematics that actually show anything from the faction that was damaged by this. All Alliance got was a portal that dumped them back in Stormwind.
Then there’s the mount shenanigans, Alliance just getting a bunch of reskinned horses for example for BFA which was something that had a lot upset at the start of the expansion. That has gotten a bit better though, as they are tossing Alliance a bone now at least with the upcoming bee mount. Still doesn’t quite make up for the rest of the shenanigans, but you know.