The values and ideals of the revolution are betrayed by the pigs and by Napoleon. The "Beasts of England" song is expressive of the original ideals and so serves as a reminder of how deeply those ideals have been corrupted by the leadership of the farm. The schism between the ideals of the revolution and the reality of life under Napoleon is consciously side-stepped by substituting the new anthem for the old.
Minimus is a propagandaist. The original song “Beasts of England” no longer applies, because it is a revolution song. Napoleon and the other pigs do not want the animals thinking about revolution, or the fact that the revolution left them no better off than they started.
When the animals start to sing "Beasts of England" after the"purges" of the animals who were allegedly allied with Snowball, Squealer stops them, saying:
It's no longer needed...Beasts of England was the song of the Rebellion. But the Rebellion is now completed. The execution of the traitors this afternoon was the final act. The enemy both external and internal has been defeated. In Beasts of England we expressed our longing for a better society in days to come. But that society has now been established. Clearly this song has no longer any purpose.
The pigs are now more interested in enforcing obedience to Animal Farm than in proclaiming the rights of animals. Hence Minimus's song glorifies the achievements of the farm and causes the animals to pledge their obedience to it.
Animal Farm, Animal Farm, Never through me shalt thou come to harm!
In Chapter VIII we learn of a poem written by Minimus that celebrates Napoleon alone in obsequious terms, celebrating him as the "fountain of happiness" and "giver of all that thy creatures love." It is placed alongside a portrait of Napoleon on the barn wall opposite the Seven Commandments. This makes it clear that, as the first response says, obedience to the farm is obedience to Napoleon.
The pigs want to replace the first anthem with this one because the first anthem was not about Napoleon. Napoleon is creating this cult of personality in which everything has to be about him. "Beasts of England" is all about all of the animals and escaping the way humans are exploiting them. Minimus's song is all about Napoleon. This shows that the revolution is no longer really about the principles of animalism and more about Napoleon himself.