In the south Slavery spread rather than grew because it was an agricultural rather than industrial form of capitalism, so it needed new lands. And slavery spread because enslaved African Americans were forced to migrate.
Slavery did not become a force in the northern colonies mainly because of economic reasons. Cold weather and poor soil could not support such a farm economy as was found in the South. As a result, the North came to depend on manufacturing and trade. Trade was the way colonists got the English goods they needed.
