Orwell has a great deal of trouble with the idea of a communist society and feels that one of its greatest problems is that of the basic trend of using the common man as the engine for change but then ignoring the needs and rights of common people once the change has been enacted. As such, he feels that the common animals are going to give the best perspective to teach the allegory he wishes to teach.
A second reason was simply that of an appeal to his audience. He knew that the story could be read and enjoyed as simply a story, but he also knew that he could touch a nerve in the way that common people were afraid of communism. It was something that was held up ideologically, but the real fears of the populace were not always articulated and this was a fantastic way for them to visualize those fears and obviously created a great appeal to that audience.
A third may very well have been the fact that so many other authors have been successful with stories written from the point of view of common (or uncommon) animals. If you look at Aesop's Fables, you can understand that appeal and perhaps get insight into why Orwell would choose the same perspective for his story.