Because they traded a lot
The Aztec and Incan people had never come in contact with the bacteria and germs the Spanish brought with them to The New World and so their immune systems did not have any specialized defense against them, this meant that the bacteria and germs the Spanish carried with them, while not being enough to affect the Spaniards, were practically overexposure for the Aztec and Incan people.
Manufacturing became more important than agriculture which led to South losing its power and wealth since people were switching away from cotton usage while North only grew even more because the products made were produced more then ever due to technological advancements.
The start of WW2 strengthen the economy by manufacturing new weapons for american troops:
