Why does Napoleon order the animals to stop singing "Beasts of England" in Animal Farm?

Napoleon says that “Beasts of England” was a song of the revolution and was no longer needed.  The real reason he bans it is because he does not want another revolution.

Old Major, the revered old boar, has a vision for England which the animals transform into Animalism under the banner of the patriotic tune, “Beasts of England.”  It’s their rallying anthem throughout the revolution, and they enjoy it afterward.  It includes such stanzas as this.

Riches more than mind can picture,

Wheat and barley, oats and hay,

Clover, beans, and mangel-wurzels

Shall be ours upon that day. (Ch. 1)

The animals are inspired by this tune to work harder, kick out the man, and run their own farm.  Under this new farm, they will have all the food they want, the song reminds them. They have been oppressed, but they will soon live a life of luxury. 

Napoleon and the other pigs allow the song to continue for a little while after the revolution, in order to keep the animals motivated.  It is sung at meetings, and soon the tune is known throughout the land.  However, soon they tell the animals to stop.  They give a good, sound, logical reason (the pigs are good at those).  The real reason, though, is that “Beasts of England” begins to be rebebllious again, and the animals are rebelling against the pigs.  When no one dares to speak his mind, Clover sings “Beast of England.”

The other animals sitting round her took it up, and they sang it three times over—very  tunefully, but slowly and mournfully, in a way they had never sung it before. (Ch. 7)

Napoleon tells the animals that “Beasts of England” is banned because it “expressed our longing for a better society in days to come” (Ch. 7).  Since now they have the perfect society they longed for, it is no longer needed.  Of course, this is quite the opposite of the truth.  The song has been revoked because the animals are well aware that they were sold a bill of goods, and their society is far from perfect.  They traded one corrupt master for another.  The new boss is just as bad, if not worse, than the old boss.  The pigs realize this, and that is why they forbid the song.