Why did Animal farm considered an allegory?

From Shmoop Literature on George Orwell's Animal Farm

http://www.shmoop.com/literary-device/literature/george-orwell/animal-farm/genre.html

Genre

Satire Satire squared, actually. The satire to end all satires. You get the point. To be more specific, the tale is a satire of the Russian Revolution. How, you say? Shmoop on, shmoopster, and check out "Symbols, Imagery, Allegory."

http://www.shmoop.com/literary-device/literature/george-orwell/animal-farm/symbols-imagery-allegory.html