Be careful putting absolute trust in your leaders.
The Animals learn that power corrupts. The rebellion was ultimately unsuccessful, because most of the animals are enslaved at the end of the book just as badly as in the beginning. They have been manipulated and brainwashed by the pigs.
Old Major’s dream was for the animals to be their own bosses, and have control over their own destiny. They were able to overthrow the humans, but almost immediately the pigs began taking control for themselves. They stole from the animals and then began treating them oppressively.
At the end of the book, the pigs are indistinguishable from humans. They are just as abusive and in many ways more reprehensible, because they are exploiting their own kind.