How did the spanish so easily defeat the Aztec and Inca?

quite simply, there had three or so things going for them

the first one is minor, but they had more advanced technology compared to the Inca

the second is MAJOR: they had a virus in their system that they were immune to, but the Inca were not.

the third is minor, would not have changed too much, but the Inca came out in celebration instead of in war, so they were quickly mowed down, and their leader was captured and exicuted later on.