It's more like, the Alliance sees that there's too much turmoil inside the faction of the Horde that stands against Garrosh, they can't be trusted to be able to finish the job. You know, while the trolls and tauren might be ready for a fight, the proud blood elves have enough on their plate licking their wounds from the encounter with the Kirin Tor currently and Sylvanas is probably more kneen on turning dead Kor'kron into her personal army than liberating Ogrimmar. Would you trust them? Well I wouldn't. If you want the job done correctly then do it yourself, that's what the Alliance is there for.
Besides, Garrosh tried crushing our prince, as much as we don't like the brat he's ours to deal with and no red-skinned pig-man goes for him unpunished.