Why did it take so long for the Aztecs to fall?

After he arrived in Mexico City, it took Hernán Cortes about 2 years to complete the conquest of the Aztec empire. I understand that the Aztecs were weakened by their divisions and the Spanish had technologically superior weaponry and the help of local tribes who were tired of Aztec brutality, but it still seems like the conquest was almost too easy to have happened. The Aztecs had a vast empire with a lot of men at arms, and the Spanish conquistadors were vastly outnumbered. How did this happen?