Why must George Orwell choose to write Animal Farm in the form of an allegory?
Perhaps you can explain the conditions of Orwell's life then, why he couldn't speak up his mind freely. Also, what about the type of society which he lived in, that he have to say things with double meaning. Maybe his real message was not even about Russia alone.
One look at the dates for George Orwell's life and the country in which he lived tell us much of the reason behind the author's choice to write Animal Farm as an allegory.
Orwell, a British citizen, lived during the first half of the twentieth century, from 1903-1950. Thus, Orwell lived through both World War I and World War II. Ironically, in 1936 and 1937, Orwell had been in Spain during their civil war and had fought on the side of the communist-backed Republican governement (although Orwell fought more because of his dislike of the Fascists than any love for communism).
Given Orwell's experience in Spain, plus his experience living through the two World Wars, in which his fellow Britons were actually allies of the Russians, it seems clear that Orwell could not openly criticize the Russians without putting himself in some peril. In fact, George Orwell is not the author's real name, but rather Eric Arthur Blair. Additionally, in an apparent effort to put further distance between himself and blatant criticism of the Russians, Orwell's original title for the novella was Animal Farm: A Fairy Story.
Furthermore, when we consider that Orwell started writing Animal Farm in 1943, at the height of World War II, the author's need to employ allegory becomes more apparent (the novella was finally published in August 1945). Thus, to protect himself and his family, it seems necessary for Orwell to publish such a novella as an allegory.