As far back as colonial times, even from almost the beginning of colonization of the America's by Britain, there were stark differences being created between the Northern and Southern colonies. Southern colonies, and eventually states, were based nearly solely and entirely on farming. They were much more the producers under Mercantilism by the British of agricultural goods. Once the U.S. was formed, the Southern states continued to supply much of the new countries agricultural goods. Northern colonies, and eventually states, have a much shorter growing season, and therefore were not as prosperous at farming and large scale agriculture as the southern states. Northern states tended to focus more on the production of raw materials into finished goods, and the shipping and transportation of those finished goods to buyers, be that England and Europe or other colonies and states. Also keep in mind the way that the colonies and states were set up from a population standpoint. Northern colonies tended to be smaller in size (not counting New York or Penn. as Northern, but as middle colonies) Which meant less overall land, and a more compact population. This led to the development of cities, and urban industry (this is part of why the Industrial Rev. in America happened nearly exclusively in the North, as well as the eventual Great Migration during WWI). Southern states were larger in size, and therefore had more land area. This led to larger holdings of land by the wealthy planting class, which allowed for large scale agriculture. Even at the outbreak of the Civil War, the South was still far less populated, with a much more spread out population. Southern states also had very few large cities, with places like Atlanta, Richmond, etc. being seen as "big cities" but were most likely average in size in the eyes of Northerners. Also, around 70% of railroads were located in the North, and nearly all of the manufacturing centers were there as well.
In regard to slavery, the above mentioned attributes of both Southern and Northern coloines and eventually states should help explain why there were disproportionate amounts of slaves in the Southern states. Around 80-85% of slaves in the U.S. lived in southern states. Again, since those states are based on agriculture, it was much more profitable for slavery to be in the south. There were slaves in the North, but these were basically used as domestic servants, because the North was based on skilled labor, factory work, and urban agriculture. Slavery in the south was actually on a decline for a period of a decade or more until the invention of a cotton gin (again, Northern industry aiding Southern agriculture). I would argue that economics played equally as large a role in the eventual secession of southern states, as evident by the Nullification Crisis (S.C. threatened to secede over a fedreal tax on the importation of goods, which would have benefited Nothern states with there large amounts of industry.)