The reason? Easy! Thousands of warriors from the many oppressed tribes who were forced to deliver human sacrifices to the Inca and the Aztecs, JOINED the Spaniards to overthrow them.
Wouldn't you?
Pingo
Yes --- people like to think the Indians were one big happy family --- but the Indians of Mexico were eager to help the Spaniards replace the Aztecs. The Aztecs weren't into self-sacrifice after all --- it was the Indians they captured from other groups that got the honor.