Major has too idealistic and an abstract idea of the perfect society, and he has not explained how it is to be put into practice.
The rebellion comes unexpectedly, without giving the animals the opportunity to work out a really democratic system.
There is a wide difference between the intelligence of the pigs and the other animals, that the pigs begin to control the society without opposition. The remaining animals are too stupid to realize what Napoleon and other pigs are doing to them.
It also fails because Snowball cannot compete with Napoleon’s brutal and selfish purpose to gain power. Napoleon is selfish and power-hungry. He concentrates his energies in winning power
It also fails because the other animals are either too stupid or too weak to fight Napoleon, or they are too vain and selfish to contribute to their society. Several animals (Moses the Raven, the Cat, and Mollie) do nothing to help the revolution, and so contribute to the failure.