Why did the native rulers treat European explorers like Cortez and De Soto with such respect?
Unfortunately, we can not know for sure why this was because the native rulers did not leave any written accounts to tell us why they acted as they did.
The usual reason given for this is that the native rulers believed that the Europeans were gods or at least the emissaries of gods. In the case of Cortez, for example, there was a myth that said that the god Quetzalcoatl had been driven out of Mexico years before and would return from the east. He was often depicted as light-skinned and bearded. Moreover, he was associated with the shape of a cross.
The story goes that the Spaniards were seen as representatives of Quetzlcoatl because they came from the east, were light-skinned and bearded, and carried crosses. Therefore, the rulers treated them with respect.
But this is a story told by the Spanish themselves. So we can not know for sure that it is a true representation of what the leaders thought.
Scholars today tend to argue that the Quetzalcoatl thing was totally made up by the Spaniards. They tend to argue instead that it was personal indecisiveness and/or curiosity that made Moctezuma allow the Spaniards to land and advance towards Tenochtitlan.