Revolutions occur when the lower classes finally become frustrated with the system as it exists and they decide to do something about it by rising up against it. This is what happened when the animals took over the farm - they rose up and rebelled against the farmer. Then, after a brief period of socialism and equality, the pigs took over creating a system that was not all that much different from the one that had existed under the farmer's regime. Examining the situation, then, from the lower animal's perspective gives Orwell the chance to fully examine what leads to oppression and why socialism is a good idea in theory but not such a workable concept when put into practice. The animals parallel through their actions the situation in the Soviet Union at the time and act as allegorical representations of major players in the Russian revolution.
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.
I assume that you are asking why he does not look at things from the pigs' point of view. I see the following reasons for this.
First, I think that Orwell identifies more with the oppressed than with the oppressors. I think he sympathizes with them and tells the story from their point of view because of this sympathy.
Second, I think that Orwell wants to explore the impact that this sort of system has on the little people. I think he is most concerned with what happens to the regular people when a system like this is put in place. By telling the story from their point of view, he highlights this impact.