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.
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.
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.