Why are all the failures blamed on Snowball?

After Napoleon sucessfully drives Snowball from Animal Farm, it becomes important to Napoleon's propaganda aims to demonize Snowball as often as possible. In reality, Snowball was a capable and popular leader, more capable than Napoleon, so it matters to Napoleon to keep denigrating him. He certainly doesn't want the other animals campaigning for Snowball's return or comparing him unfavorably to Snowball.  Second, Snowball is a convenient scapegoat. He isn't around to defend himself, so it is easy for Napoleon to blame his own failures on so-called "plots" by Snowball. When Napoleon wants to get rid of animal enemies, saying they were incited by Snowball is a foolproof way to demonize them and then have them executed. In his tactics, Napoleon mirrors Josef Stalin. Stalin exiled his rival Leon Trotsky, demonized him, and then in show trials in the 1930s, blamed him and his alleged followers for any number of problems suffered by the Soviet regime.