The Horde have just come out of a damn civil war. They are not at their strongest atm. Whereas the Alliance are apparently more united than ever and didn't suffer as many casualties anyway, especially after their ploy in 5.3 to let the Trolls take the fall.
On the flip side though it gives the Horde a story right out of the bat in the next expansion as they have rebuild themselves in the face of their civil war.