There is no one answer to this question as there are a number of contributing factors involved. Much depends on the political opinions of each individual.
Snowball, for example, would say that the revolution failed because Napoleon became a dictator and refused to spread the Animalist revolution beyond the confines of Manor Farm. And look what happened! Napoleon, like the rest of the pigs, has become almost indistinguishable from a human. Animalism was supposed to liberate animals from humans. But now, thanks to Napoleon's lust for power and ideological ignorance, animals have become every bit as repressive and exploitative as the humans.
Farmer Jones, representing humans in the story, on the other hand, would doubtless say that the revolution failed because it was a stupid idea in the first place. Whoever heard of animals running a farm? He would not be very surprised the place has been mired in chaos, bloodshed, and chronic inefficiency since the animals took over.
Boxer, if he had lived to tell the tale, might have offered his own unique perspective on why the revolution failed. The revolution was a noble cause, one that had the potential to liberate the animals and give them control of their own destiny. Although not everything went according to plan, the revolution did nonetheless give the animals a taste of just how important they are, a real sense of their own worth. Boxer might say Animalism isn't dead, that Manor Farm must keep its flame alive in the hearts of millions of animals worldwide through ceaseless struggle. He would urge others to aspire to build the Animalist utopia, only this time learning from the past mistakes of some of their less ideologically correct former comrades.