Among other things, their racials are hard to go wrong with (they’re all somewhere between good and excellent), the cultural zeitgeist has always favored Horde over Alliance (they’re “cool” and “badass” while Alliance are “boring”), and the e-celebs with big mindshare all play Horde.
And that imbalance was created by all the players who participate in high end content transferring. Alliance is currently full of players who never did that stuff or only dabble because they don’t minmax and don’t see any point in transferring.
It’s not impossible to turn these people in to high end content participants, but it’s going to be an uphill battle. There’s a significant rift between players who raid on heroic and higher or do M+ and those who do not.