I think that there are a couple of things to note on this particular question. The first would be that Orwell's own experience made him a flagrant opponent to Stalinism and the brand of Communism that was happening in Russia at the time. His loyalty to the Trotsky ideas led him to flee from Spain in his participation against the Franco- led forces in the Spanish Civil War. Orwell's anger and resentment at the betrayal that was inside him regarding the Soviet leader and the twisting of Communism that was being featured in the Soviet Union compelled him to write a work criticizing the Russian leader and the brand of Communism being advocated. At the time of writing the book, Orwell lived on a farm and much of the animals that surrounded him were seen in the novel. The use of the farm as a setting became a point in which artistic commitment and practical reality converged into the form of the novel being written. Seeing that Orwell needed to find an allegorical basis for the novel, the farm seemed to work extremely well and setting it as the basis for his critique of Russia seemed to make sense.