Alliance wants to do nothing of the sort, while Vol'jin is the one that wants to kill garrosh and invade Orgrimmar. Alliance can just sit back, relax, and watch as the Horde decimates itself from within, and then just swoop in and finish them off in one swift decisive strike, while they are still weak.
So my question is: Why should WE help YOU? (except loot.) What do we win? Alliance are commiting SOLDIERS, SUPPLIES and HEROES to help the HORDE to change their moronic leader, wich, given he stays in power, not only will greatly weaken the Horde as a whole, but would grant the alliance the full support of the Pandaren in Pandaria. What LOGIC is there BEHIND this!?