When the original colonists came to America, what they found in the North was rocky land that was not very conducive to mass farming - virtually all Northern farmers were subsistence farmers, farming for their families and their villages. Therefore, they never needed many slaves (Northerners did have slaves). In the South, the land was great for huge plantations, where the plantation owners needed cheap labor, and got slaves. By the time the North industrialized, and could have used slaves on factories, the Northern states had already abolished slavery for a long time, and therefore wouldn't try to reintroduce it. Meanwhile, the South never truly industrialized, and continuing to use slaves just economic sense as the economy was still based on huge plantations.

Australian here, going through a fair bit of US Civil War reading. All the books start with the Mexican War (or thereabouts). What I can't get around my head is why, in 1820s US, the South and the North are already such very radically different societies and economies. How do states in Union differ so very dramatically, especially on such a big difference as slavery.

PS - the whole antebellum politics from the Mexican war onwards is absolutely fascinating. So many parallels to the reading I did on the first world war - this monstrous thing on the horizon which so many acts foreshadow by decades.

Partly it may have been due to land use. Southern farms could grow almost the whole year, and so there was a lot of use for slaves all the time. Northern farms had long months when it was cold and the land lay fallow, meaning there wasn't a lot of work for slaves but they still had to be fed. This didn't make economic sense.

It was also ideological--it was northern statesmen who complained about the inconsistency of demanding freedom from England yet stollen salving people. So Northern states started emancipation slaves, state by atate.