why do the animals submit to napoleon's domination?

Before answering this question, we should remember that all the animals have ever known (for their entire lives and the lives of their parents, grandparents, etc.) is the rule of humans. After the battle against the humans is won and the animals are free to rule themselves, life is completely and utterly foreign. The animals have lost any and all security, so they are badly in need of a stable form of leadership. When Napolean successfully ousts Snowball, then, there is really nowhere else to turn for this dire need.