Why did Orwell choose a farm as the novel Animal Farm's setting?

In his 1947 preface to the Ukrainian edition of Animal Farm, Orwell provided some background on what led him to the farm setting of the story. He was distressed at the way many people in England misunderstood the severity and cruelty of what was then occurring in the Soviet Union under Stalin. Orwell did not think that Stalin's brutal totalitarian regime should be confused with true socialism. The problem of how to communicate the reality of what was going on in Russia plagued Orwell. He recounts:

[F]or the past ten years I have been convinced that the destruction of the Soviet myth [the myth that it was socialist state and not a dictatorship] was essential if we wanted a revival of the Socialist movement.
On my return from Spain I thought of exposing the Soviet myth in a story that could be easily understood by almost anyone and which could be easily translated into other languages. However, the actual details of the story did not come to me for some time until one day (I was then living in a small village) I saw a little boy, perhaps ten years old, driving a huge cart-horse along a narrow path, whipping it whenever it tried to turn. It struck me that if only such animals became aware of their strength, we should have no power over them, and that men exploit animals in much the same way as the rich exploit the proletariat.

This gave Orwell the idea of setting his story on a farm, among animals.
A farm is simpler to describe than an entire nation state and so made Orwell's story easier to tell. Animals provide enough distance from the human experience that we can understand Orwell's main points more easily. He wanted to show how easily the ideals of a revolution can be corrupted by the misuse of language and by violence. The farm, with its various types of animals, gave him a simple stage on which to place his ideas about the mechanisms through which one group or individual twists ideals and turns them to evil.