So check it. In 1776, the thirteen colonies had a combined population of 2.5 million people. The population of England, Scotland and Wales was about 5.5 million people. Nonetheless, English law forbid sales of heavy machinery or manufactured goods from the overseas colonies, and English involvement in the Seven Years' War was in no small part paid for by taxes on English goods sold to said same colonies.
The South mostly gave no fux about this because they couldn't manufacture shit. They weren't set up for it. The North, on the other hand, citified early and often and tended to attract obnoxious assholes like Ben Franklin who don't know when to stop thinking. This is why when you learn about the Revolutionary War, you mostly learn about Paul Revere and the Tea Party and the Liberty Bell and shit distinctly north of the Mason-Dixon line. South went along with it because hey, if you need something manufactured, a third of the population of the British Commonwealth isn't an ocean away and turns out Americans were, in general, better mechanics than the English (much to their chagrin - fuck off, Worshipful Company of Eat a Dick.
This is Wallerstein 101 - the core is where innovation happens, the semi-periphery is where adaptation happens, the periphery is where resources come from. The American South was, right up to the Civil War, periphery - it produced raw materials which were exported in trade for manufactured goods. The American North was, from like the early 1700s, semi-periphery... they just weren't allowed to export, so they went to war. A lot happened in the ensuing hundred years or so as far as protectionism and such but in the process, the North became a manufacturing hub, trading refined products rather than raw materials.
A semi-peripheral state will either slide back into the periphery or it will advance to the core. If you're in the core, you don't want competition. If you're in the semi-periphery, you don't want to descend back into the periphery. You advance towards the core by focusing on innovation, manufacturing, research, skilled labor, training and all that other Western Industrial Might shit that turns "korea" into Korea. (side note: this is one of the many reasons Japan invaded everything within reach during the Meiji Restoration: you don't want to compete on your own terms, you want to dominate your neighbors like you're the British East India Company).
So. The Northern economy was based on industrialism, and its future depended on increasing industrialism. The South, on the other hand, was based on agriculture... and even that was experiencing severe change. Slaves can't maintain cotton gins. You don't wanna send your chattel to school because they might learn how not to be chattel. The North was arrayed around "everyone's free, eventually you too can strike it rich if you work hard enough" (true or not, that was the ethos) while the South was arrayed around "my family has owned this valley for six generations how dare you insult my honor, sir." Both societies were class-conscious in that if you owned shit you were on top but free Southerners were easier to keep happy because no matter how shitty your life was, there was always someone with a shittier life than you, so there was very little white impetus to change the status quo.
So. The Northern economy goes from "craftsman" to "industrialist" and it needs skilled labor. The Southern economy goes from "slave owner" to "slave owner" and the minute you inject skilled labor into that mess, you have a middle class to deal with and they don't like that no matter what they do they'll never own a plantation so you want to keep it out. 'cuz you know what? As soon as you have a social class where poor white people can make something of themselves without the direct patronage of rich white people, they start aligning with the black people against their mutual abusers.
I don't want to give the impression that Northerners hated slavery for purely economic reasons. Slavery was and is morally inexcusable. I do want to point out that most people give more of a fuck about the moral trespasses of others when it directly impacts their livelihood and the fact of the matter is, the economies and societies of the Northern and Southern states were drifting apart, the Southern states provided useful inputs to the British economy and the longer the United States was allowed to develop, the more of a threat to British hegemony it became.
The South was never going to become an industrial society on its own. It was a feudal agrarian society whose wealthy profited off of the labor of vassals. The North was either going to succeed or fail based on its industrialization. It was a rapidly industrializing society whose wealthy profited off the labor of craftsmen and artisans. Something had to change. It wasn't invariably going to be war, but war it was.
As an aside, while this situation has effectively nothing to do with the modern United States (Boeing just announced last week they were moving their headquarters from Chicago to Virginia, FFS), it's topical when discussing Russia and Ukraine. The tension there has always been that Russia is an agrarian society where wealthy landowners used slave labor to accumulate wealth while Ukraine was a peripheral state whose economy largely traded with Europe and the Ottoman Empire. Russia has been shithole steppe going back to the Parthians whereas Ukraine has bounced through a dozen different kingdoms, all of which are more firmly anchored to Europe than Russia.