When the "laws" on the wall keep on being changed, all the other animals except the pig never question why that happens even though they know that something different being put up there originally.
Also no one seem to remember that strength in numbers is what got rid of the farmer in the first place, and this idea could also get rid of the tyranny of the pigs. However they were too ignorant to perceive that the pigs were oppressing them and trying to brain wash them.