It’s partly because the Alliance are sort of written like an empire. There’s a lot of contradictions in how both factions are written, and a longtime issue of “What Blizzard says is lore, and what they actually do in the story” (without even touching on the times when they blatantly lied in interviews regarding subjects like Garrosh or Sylvanas).
A TL;DR of longer discussions like this one is that the subfactions of the Alliance are labeled as a collection of races coming together, but then effectively act as vassal states of Stormwind’s empire.
I personally think they’re playing into that old generic fantasy style where the humans are the leaders of everything, and then the humans have a bunch of servants/guards who are non-humans, but the only leaders are humans. It’s a trope a lot of fantasy settings tended to get mired down in, though it comes across as dated to me now.
What’s weird to me though, is that most franchises become self-aware over time, and move away from this trope. Star Wars, for instance, introduced a bunch of aliens to the Rebel Alliance in Return of the Jedi and made the Mon Calamari both intentionally ugly and prominent to show kids that “ugly people can be heroes”. You would think the franchise who cashed in big on the realization that “people want to play as good guy monsters” would recognize this by now. Granted, it sort of felt like they became self aware for a while, but then the story keeps going back to ‘Anduin/Jaina’s adventures with human form Genn, featuring sidekick race of the patch’.