Mercantilism and barter are better described as historical economic systems, and in the past the governments and the kings/queens generally kept a closer control of the country, so let's not consider those options.
Now, we have: capitalism, communism, socialism.
Capitalism leaves the capital and the economic decisions in the hands of the private people, not the government. Socialism means a common possession of the means of production, but not by the govrenments, rather by the people, as is often seen as a milder form of communism.
Communism on the other hand is associated with authoritarian governments. Some example were the Soviet Union, and currently, North Korea and Cuba.
Answer:
The government wanted to keep the number of free and slave states equal so that neither of would throw off the balance of congress.
For example, if there was more slave states in congress, then most likely the laws would be made in favor of slavery.
