Old Major's vision fails because of his own myopia. Old Major never envisioned that the desire for power and control was a reality that could afflict animals. Old Major fundamentally believed that such repugnance was a part of being human. He never envisioned that animals, themselves, could wish to display power. In broadly advocating that all of the animals could overtake the humans and implement Animalism, Old Major believed that only theoretical success could be evident.
The cause of his dream's failure was that the pigs and dogs sought power just as much as the humans did. Orwell's basic truth here is that power is a corrupting element. It is shown to infect anyone and everyone. Social orders that are driven to create a better society can only find success if such elements can be checked and limited. This is where Old Major's vision ultimately fails. He does not account for any checks or balances to his system of government. In failing to do so, a hierarchy is created where, by the end of the narrative, the animals and humans are indistinguishable from one another.