Cortes goes to Mesoamerica, discovers a great empire, and plunders it.
Pizarro goes to South America, discovers a great empire, and plunders it.
Soto, who was with Pizarro in Peru, infers that there must be a great empire to plunder in North America.
He leads a military expedition through Florida, Georgia, Alabama, Mississippi, and probably Arkansas, finds no great empire to plunder, and dies of fever on the banks of the Mississippi.
The Spanish therefore give up on North America, more or less. No native empire had done the hard work of finding and extracting the continent’s gold for them. And no native empire had done the hard work of marshaling the population into labor units that could be exploited from the top by invaders. It seemed obvious that the hemisphere would be most easily dominated from the places at which native civilization was most advanced.
What saved North Americans from the slavery that befell Mesoamericans and South Americans in the 16th century was that they had not been brought under the centralized control of their own native emperor prior to the European invasions. In the language of James C. Scott, they, unlike their Aztec or Inca neighbors, had not yet been domesticated; they were still free. And that made it impossible for the tiny groups of Spaniards who were carrying out the colonial project to dominate them. For those groups operated by killing the native emperor and substituting themselves at the top of a pre-existing power structure.