Because the legion will faceroll you if you don't. And Garrosh would just end up taking control of the horde and bomb more of your cities. And I am more willing to let him do it if the Alliance are filled with people like you...try doing things for the 'Greater good'. If you don't help us, the humans will end up dieing themselves. The moral story of mop is suppose to understand that the fighting of horde vs alliance is stupid.
He who fights with monsters should look to it that he himself does not become a monster. And when you gaze long into an abyss the abyss also gazes into you.~Nietzsche
Edit: After this all the sensible Orcs will be part of the good horde and the bad ones will end up dieing for their leader. So essentially we will be just cleaning house:}.