Why don't the pigs like the pet raven Moses' stories about Sugar Candy Mountain?

Moses is Farmer Jones' "especial pet" and is a spy. So, to start with, the pigs mistrust the raven because of this. Most of the animals hate Moses because he does not work and talks too much. But the allure of Sugar Candy Mountain is convincing to some animals. Knowing that Moses is a spy and confidant to Mr. Jones, the pigs suspect that tales of Sugar Candy Mountain are meant to convince the animals to accept their difficult lives under Farmer Jones. If they have this heavenly reward waiting for them, it would be easier to endure such difficult lives. The pigs don't want the animals to fall into this way of thinking because they might then prefer Jones to rule the farm again.

Note that the pigs represent the Communist Revolution in Russia. Stalin (Napoleon in the novel) allowed the continuance of the Russian Orthodox Church, but he eventually became an atheist. This would also become the official Communist party line. The pigs allegorically follow this trajectory in the novel. They feel that the notion of Sugar Candy Mountain is a strategy used by Jones to keep the animals ever hopeful in a final reward, despite their struggles in life. The pigs want to eliminate this notion from the animals' minds. They feel it is more important to focus on their own revolution rather than this potentially false promise of Moses/Mr. Jones.