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.