Napoleon usurping power and utilizing Squealer to manipulate the other animals is the primary reason the revolution is a failure. Initially, the animals successfully expel Mr. Jones and his men from the farm and establish an egalitarian society founded on the principles of Animalism, which is a system of thought derived from Old Major's speech championing animal independence and equality. Shortly after establishing Animal Farm, Napoleon, with the help of his nine ferocious dogs, chases Snowball off the farm and usurps power. Napoleon immediately stops accepting proposals from the other animals during the Sunday meetings and assumes complete authority. Napoleon then employs Squealer to manipulate the animals into supporting his political agenda while simultaneously altering the Seven Commandments to coincide with his policies. Napoleon and Squealer use Snowball as a scapegoat, confuse the animals by making illogical arguments, and establish policies that oppress the other animals. In doing so, Napoleon creates a pig aristocracy and forces the other animals to labor all day.

Napoleon also cultivates an atmosphere of hysteria and fear by holding public executions and surrounding himself with his intimidating dogs, which allegorically represent Stalin's police force, known as the NKVD. Napoleon also establishes a cult of personality, and the majority of animals revere him as their courageous, intelligent leader. Under Napoleon's leadership, the animals live depressing, arduous lives, and the working conditions on Animal Farm are significantly worse than they were under Mr. Jones's reign. By the end of the novella, Animal Farm has completely transformed back into Manor Farm, as Napoleon simply replaces Mr. Jones. One could argue that if Napoleon had never usurped power, the revolution might have been a success.

In Animal Farm, the revolution fails because its emphasis on equality and community is destroyed by Napoleon's self-interest and desire for power. To demonstrate this, take a look at the difference between Chapter Two and Chapter Ten. In Chapter Two, for instance, the animals are united by the Seven Commandments, which vilifies humans and makes all animals equal. But, by Chapter Ten, there is a clear difference in the farm's ideology. The Commandments are whittled down to a single rule: that some animals are more equal than others. In other words, the pigs, especially Napoleon, are the most important individuals on the farm and their needs and desires supersede all others.

By creating this distinction between the pigs and the other animals, the revolution has lost its founding principles. Moreover, the closing image of the novel shows the pigs (who once loathed Mr. Jones) dressed in human clothes and walking on two legs. They became the people they hated because their ambition—and not the common good—became the revolution's driving force.

The major reason why the revolution fails on the farm has to do with the way that the principles and ideals of the revolution itself were subverted by those who rose to power, particularly Napoleon. At the beginning, the seven commandments were at the heart of the decisions that were made and the animals felt they had gained some control over their destiny once they turned out Farmer Jones. 

But once Old Major dies, the care of the revolution and the leadership of the animals falls to Napoleon, Snowball and Squealer. While Snowball has the interests of the other animals at heart and works to pursue those ideals, Napoleon quickly begins to take advantage of his power using Squealer to twist the truth and the dogs to attack those who dare to resist his subversion of the ideals of the revolution.

By the end of the book, Napoleon and the pigs act exactly like Farmer Jones, showing no concern for the welfare or interests of the other animals. He and his henchmen break all of the seven commandments but by twisting the truth as they see fit and using force to stop any major objection, they get away with it.

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.

Why do the animals fail to bring about an ideal society in Animal Farm?

When answering this question, it is important to distinguish between the pigs and the other animals. For the pigs, the rebellion did bring about their ideal society: after overthrowing Mr Jones, for instance, the pigs have risen to become the leaders of Animal Farm and enjoy all the comforts of human life, like living in the farmhouse, making money and drinking alcohol. Only Snowball believed in creating a utopia in which all animals are equal and he is run off the farm by Napoleon as soon as he becomes a threat to the pigs' power.

For the other animals, their failure is a combination of several factors, of which the greed and the corruption of the pigs are certainly important. But the other animals also lack the intelligence to see through the pigs' propaganda, as shown by Boxer's death at the hands of a glue manufacturer. The fact that Squealer convinces them that Boxer was not slaughtered also demonstrates their lack of common sense: they cannot see that the pigs do not rule for the benefit of everyone, only for themselves.

Why do the animals fail to bring about an ideal society in Animal Farm?

The animals are constantly misled by the pigs who steal the farm's resources for themselves. The pigs are the only ones who can read (though Benjamin the Donkey might be able to, the reader is never sure) and they change the commandments of Animalism to whatever will help them the most. The pigs use martial speeches, military-style songs, and fear in order to convince the animals that the revolution is ongoing and it is up to the pigs to provide leadership. The pigs put down rebellion brutally, such as when the Minorca hens are slaughtered because they destroyed their eggs rather than give them up for sale. The pigs also claim a boogeyman for the farm, Snowball, and blame all of the farm's problems on him when in reality the farm's problems are due to the pigs' mismanagement. The animals did well in overthrowing Jones, but without leadership, the revolution comes full cycle, and by the end of the story, the pigs are acting like humans.

Why do the animals fail to bring about an ideal society in Animal Farm?

The utopia which was the original purpose of the animal rebellion did not become a way of life because of greed, arrogance, and power.  The "pigs" decided by the end of the novel that they were "better than all the other animals." They became rich from the labor of the other animals on the farm.  Many of the original members of the rebellion were gone or dead and the pigs began acting more and more like the humans they said they hated.  By the end of the novel the cold, hungry, and overworked farm animals couldn't even tell the difference between the men and the pigs.  The constant desire for power, control, greed, and arrogance ruined the Utopian ideals.