Why do the pigs in Animal Farm convince the other animals that Sugarcandy mountain does not exist?

Moses the raven claims that, when animals die, they go to a paradise called Sugarcandy Mountain. It's kind of like the animal equivalent of heaven, a place of eternal bliss. Some of the animals want to believe in the existence of Sugarcandy Mountain—and why not? It sounds like a truly wonderful place, with clover all year round and sugar lumps and linseed cakes growing on the hedges.
To the likes of Napoleon, however, Sugarcandy Mountain is a dangerous delusion which diverts the animals from building a true Animalist paradise on earth. As Animal Farm is a political allegory on the Soviet Union, we should recognize that Napoleon's negative attitude to Sugarcandy Mountain reflects the skepticism of Communists towards religion. Marxists like Lenin and Stalin regarded religion as "the opium of the people," a kind of drug that kept the masses in a state of subjection, reconciling them to the oppressive Tsarist regime. Yet the Communists simply replaced one form of religion with another, exchanging Orthodoxy for Marxism-Leninism, which became an all-encompassing secular creed.
As Napoleon is determined to establish himself as dictator, he doesn't want the animals to place their trust in any kind of otherworld or after-life. Heaven will be established right here on earth, on the farm, and Napoleon will be the all-powerful god in charge of this Animalist paradise.