It is important to think about how this parallels to the Russian Revolution.
If Farmer Jones represents the reign of the Romanov family, when Lenin (Old Major) came along, the people weren't quite sure whether to follow Lenin or not. This is very probable among peoples who weren't taught to think for themselves or the value of the freedom to think like we have here in America. They people were used to being treated a certain way, even if it was bad. So if Animalism represents the beginning tenants of Communism, the animals justifably had room for question. What's interesting about the different animals is that they likely represent different types of people. Some people just go with change when it happens. Others are completely unchanged no matter who is in charge. Yet another type of people have fundamental beliefs and values that they stick with strongly and when an ideology comes along, they either follow strongly or oppose it.
The answer to this question can be found at the very beginning of Chapter 2.
Various of the animals have different arguments for why it is either not right or not practical to follow the principles of animalism. They raise these at the early meetings on the subject.
Some of the animals argue that they have a responsibility to remain loyal to Farmer Jones. Others argue that it is not practical to break away from him -- they say that he is the one who feeds them and they would starve without him.
Others of the animals do not care, or do not care enough to help with the rebellion. They think it will either happen after they are dead or they think it will be able to succeed without them so they have no incentive to participate.