In Animal Farm, why did Orwell write a fiction novel instead of a political essay?
Styling the story as an allegorical "talking animal" tale allowed Orwell to reach more people. If he had written a political essay, it would have been read by those with political interests and ignored by others; as a fable, the story is very widely read and understood even by those who would ignore the same events in real life. Each character in the story represents a person or organization in the Russian Revolution, and so the parallels are easier to understand.
"Is it not crystal clear, then, comrades, that all the evils of this life of ours spring from the tyranny of human beings? Only get rid of Man, and the produce of our labour would be our own."
(Orwell, Animal Farm, msxnet.org)
Instead of covering thousands of events and decades of history, the book distills its message into a smaller space and shorter timeline. This gives the story speed and allows difficult concepts -- such as the secret police and informants -- to be explained in easier terms. However, the story is not meant for children; it is brutal and bloody, and has quite an unhappy ending.