A lot of it seems to be what I call the plebs. No gear no understanding or wish to play objectively or no understanding of objective play. they usually display one or more of those traits. Horde has none of that, horde has a smaller population of players where most all are at least semi competent.
Also the class balance. Horde ALWAYS has healers queuing disc/rsham alliance is lucky to get 2 healers in a game and it's rarely an raham it's usually 2 discs. Alliance has plenty of rogues warriors and hunters vs 4 boomkins they usually suicide into mid and get gy farmed.
gear might have something to do with it since nearly all casters can be an int or two away from bis just horde and alliance has most all the melee specific gear