In Animal Farm, how does Orwell show that a successful revolution is unlikely?
The purpose of a revolution is to foster real change in government and society, to eliminate corruption, and to create a new paradigm that is objectively better than the one it replaces. In Animal Farm, the revolution appears at first to be successful, but is shown to be a simple changing of leadership while keeping the same -- or worse -- conditions for the animals. The famous last line reads:
The creatures outside looked from pig to man, and from man to pig, and from pig to man again; but already it was impossible to say which was which.
(Orwell, Animal Farm, msxnet.org)
This line showcases the classic saying "Meet the new boss, same as the old boss." In other words, the leaders have changed but the situation remains the same; the animals are still working mostly for the benefit of others, without gaining the comfort and better conditions that they were promised by Old Major's revolutionary rhetoric. Because the pigs use their intellect to control the farm, they move into the position of surrogate humans, but are more directly exploitative of the animals. If they had remained with Farmer Jones, they would continue to serve him, but at least he did not lie to them, pretending that he exploited them for their benefit.