1. The Spanish understood the abundance of natural resources and the economic advantages of having a colony or settlement in the region. They also noticed that the Aztecs were technologically inferior to them and used this to their advantage in order to acquire the territory.
2. Race, culture and religion often go hand and hand so they most likely saw the Aztecs as being inferior in general and an obstacle to getting what they wanted. Also, the Spanish were trying to convert as many cultures to Christianity as possible because they felt like it was their duty as Christians. This is a major factor in how they oppressed Aztec culture and religion in order to push their own agenda.