I think to an extent, this is where they want the Horde. I’ll try to explain what I mean, but bear in mind I am talking from a roleplayer’s perspective.
Going from the start, it was always described as pretty much an alliance of convenience, Orcs, Trolls and Tauren pretty much had to cooperate or die, until they grew strong enough. Later the Forsaken joined, for the same reason, they were not accepted by the rest of the world, even their own former people. Then the Blood Elves, again, a weakened, battered people who needed the strength of a coalition to help them rebuild, the Bilgewater were in the same boat (or would have been if the Alliance hadn’t attacked and sunk it 
) only the Huojin really had a choice in the matter, and lets face it, whilst Ji Firepaw seems like a fun guy to hang out with, I wouldn’t trust his ability to make thought out, well informed decisions, but hey, I mean that is part of the Huojin philosophy I guess.
The one thing, ironically, out of a coalition of Species who banded together for Strength, is that it can never stay stable when -one- Species becomes ‘too strong’. The Orcs became too strong under Garrosh, and the Horde started to break. The Blood Elves started to become strong enough in their own right, and promptly considered seceding from the Horde, the Forsaken became too dominant under Sylvanas, and so her policies became much more prevalent, policies that the rest of the Horde had trouble reconciling, so again, it started to break, and ultimately, has.
That’s the irony. For an organisation that prides itself in -Strength-, it also seems very distrustful of someone gaining too much -Strength-.
Perhaps it is a deep seated inferiority complex, they remember being oppressed/misled/dominated/weak, and so they have an inherent mistrust of someone who gains too much power over them. They prefer leaders like Thrall, or Vol’jin, who did not show such supremacy as to set alarm bells ringing, but took a much more collaborative approach, I mean lets face it, Vol’jin said it best. “The Horde is Family”. In a family, once the kids are grown up, everyone is pretty equal (except in a few societies)
Perhaps it -is- that simple, all the races have grown up(regained their strength), and -could- fly the family nest when they wanted, so someone trying to tell them what to do will never go well for long.
I think that’s what Blizzard want, for the Horde always to maintain that feel of an Alliance of Convenience.