why ere the pigs immediately accepted as leaders?

Probably they were immediately accepted because the original ideas came from Old Major. As an old, well-respected boar, he was looked up to and had the ideas about what the future should be like. Once the Battle of Cowshed was finished, the pigs seemed to have the best ideas for how to be organized and how to set up the work schedules. They took the reins and then made plans for the rest of the animals.

There are a few reasons the pigs were accepted as leaders. The visionary of the entire revolution was a pig, Old Major, who outlines his dream of a world free of humans at the very beginning of the novel. After Old Major's death, the pigs, who are described as "the cleverest" of the animals, organize Old Major's thoughts into a coherent belief system they call "animalism". The three most important organizers are Napoleon, Snowball and Squealer. Snowball is the one that paints the seven commandment on the wall. Thus, by taking leadership early, the rest of the animals, who are not used to being leaders themselves, are easy targets for the pigs who seem clever and have the other animals' interests at heart.

Source(s): http://www.enotes.com/animal-farm/q-and-a/why-were-pigs-immediately-accepted-leaders-57351