There are literally entire degrees a person can get on this exact question.
Tl;dr: climate differences made the south ideal for plantation farming. No regulations made chattel slavery the best (debatable) way to farm. Large-scale farming wasnt profitable in the North. FF ~ two hundred years of this and the south becomes a racially striated Aristocracy whose entire economy was supported by cotton (see: "King Cotton") and the north errs closer to industrialization. FF another hundred years and more and more slaves are escaping the South towards the North, influencing public opinion towards abolition, which provokes the south to go even harder into the race stuff.
Large-scale farming wasnt profitable in the North.
I'd like to point out that large-scale farming was very profitable in the North - just not of cotton and tobacco. Northern grain was as important an export as Southern cotton - but grains are generally more land-intensive, while tobacco and especially cotton are more labor-intensive (thus making use of slave labor profitable).
You can also see the difference you mentioned between the majority of the South and Appalachia, which remained a stronghold of Unionist sentiment. Appalachia is terrible country for massive cotton plantations, so slaver power never really set down deep roots there.
It's mildly, mildly annoying that I know you mean the USA simply because you don't specify which country.
Well, it took me a while, for some reason I first assumed Korea.
I had the idea of going "which country? Korea?" but I thought I'd be a bit more forthright.
I thought Sudan
Slave labour was in high demand in the south due to the types of crops grown there. So, it didn't become different. The economics were just different to begin with.
