The Alliance is also made up of traditional good-guy High Fantasy races: humans, dwarfs, elves, and shortfolk. The Horde? Orcs, ghouls, minotaurs, and trolls - traditional bad-guy races. WoW morality might be blue-orange (or should I say blue-red?) but there might be some imported perception bias.
From the outside they may appear good, but their actions are hardly righteous.
The thing is, the Horde are far from evil in my opinion; and which is why I dislike them so much. They may be brutal and ruthless, but they don't go around eating babies and they're hardly depicted as a major threat to Azeroth (unlike, say, Deathwing or Arthas were). In fact, many times they're displayed as the victims (Turajo being a prime example) while the Alliance is depicted as unreasonable bigots. But then they go right along and pull some stunt against the Alliance which makes them seem sort of evil in a way.
Honestly, from an outside perspective, there is a "good" side and an "evil" side. But once you get into the game's story, you can see they're both dirty and corrupt in their own ways.
And judging from WAR and TOR, a bias against good.
Yeah, it's always been a sort of annoyance for me that it's sort of the "cool/popular" thing to play as a bad or misunderstood character and playing a character that appears somewhat noble or righteous is frowned upon for some reason. But whatever toots your own horn, I guess.