why is animal farm a credible allegory of human political life?

It is difficult to know the possible criteria for a "credible" allegory, but Animal Farm seems to have quite a secure place in the canon of the capitalistic West due to its sharp satire of the former Soviet Union. Although the characters are farm animals, there is a sense in which the "fairy tale" style speaks to some profoundly human truths (think Napolean's ruthlessness or Boxer's loyal stupidity). The Communist Revolution of the twentieth century does have both a comedic and tragic side to it, and Orwell seems to have captured that well in the novel.