IOTL the Americans were not immune to the virus from the old world, which caused them to be conquered easily after an epidemic which Severely weakened their society.
Could it be possible that an earlier colonization from the old world, which brought enough population to make the spread of old world diseases possible, failed to take advantage of the situation and conquer the new world, but instead pulled back from the new world to give Native Americans who survived the epidemic qnd had the immunity to rebuild the civilizations without external interferences?