The South was highly agricultural and thus wanted/needed a cheap and big labor force.
It wanted to keep slavery as a way of life on their plantations.
Now for the North it was much different. It was far more industrialized than the South, and so they didn't feel like they needed as many workers or slaves to keep making money. They saw this as unnecessary and increasingly morally wrong.
The Missouri Compromise is also another reason why so many of the Southern states were slave states.
