The Alliance has had plenty of problems (and still does). Most races distance themselves from the splinter factions that get a little too hell-bent on destruction (Dark Iron, Grimtotem, Gurubashi, etc...) However, those groups are largely individual tribes, and when they go crazy they just lose their connections with the other tribes.
The orcs used to be individual tribes that only came together once a year. The demons organized them all under a leader they knew they could control. Since then the orcs have really only had one leader. It's not like Garrosh is the leader of the Hellscreams, but the Frostwolves can just stay under Thrall and do their own thing. Hence, the need to kill their leader and get a more respectable orc in there! :P
---------- Post added 2013-03-25 at 11:24 PM ----------
"To learn who rules over you, simply find out who you are not allowed to criticize" -Voltaire