The US economic system is called a mixed economy because it includes not just private companies but also many government-run enterprises. Some of the latter enterprises are; the Export-Import Bank of US, Corp for Public Broadcasting, Federal Home Loan Banks and the Legal Services Corp for example.
The Northern states were against it because they banned slavery, which, as a result caused them to not have a large population of slaves. The Southern states had more slaves as a result of importing them from foreign countries so the population was greater in the Southern states then the northern ones.