There was a power vacuum that had to be filled. Any time there is a group in which no one is currently in power, someone will step in. This is a unfortunate truth of human nature. In the case of Animal Farm, the pigs were able to step in right away because of a generally understood stereotype of pigs. They said they were smarter and so the rest of them believed it was true. The animals needed to be led by someone, and the immediate response was to follow the truths that Old Major had already presented. Since he was a pig, it was natural to follow the lead of the group who would be most closely associated with this leader.