Only the importation of slaves banned, for the entire country. Slavery died in the North for economic reasons; the agriculture season was short, and so were the business cycles. It was easier to sucker desperately poor Europeans over and then fire them when business slowed down and let them starve to death or die from the winters, since there were always a fresh supply coming over. Salves in the South had it pretty good comparatively; they had shelter, could grow as much food as they wanted, and got shoes and clothes. The latter generated a boom in northern textile mills and shoe factories.