The problem is that the Horde races are struggling ones, that thrive because they have banded together.
The Alliance doesn't care about them thriving. They do not care for giving them a place in the Alliance. And they definitely do not care for making investment and personal sacrifice towards the good of these races that they would never accept as equal citizens in places like Stormwind.
The moment the Alliance wins, they stop being the "Good guys". Because despite the empathy of Alduin, Orcs, Forsaken and Trolls would never receive the care, empathy, acceptance and support of the average Alliance citizen.