So, what was the cause of this gradual decline? Well, some things just don't happen all at once. If it was climate change, the climate was probably getting more drier over the years that have passed. If it was because of the rulers, people probably got fed up with all the unfair rules over the years. And if it was caused because of the shortage of crops, people probably passed away since there wasn't enough food for them or they probably moved to another area in search of good climate. Their army couldn't match with the Spanish conquerors. They had a larger number of people and they had better weapons.