The fact of the matter is that slavery was negotiated endlessly. The north and south argued about it ferociously, and there were dead ass fist fights in Congress about it. Remember that the US was hardly the first, and closer to the last, nation in the west (I’m only making this distinction because I know significantly less about this time period outside of the west) to ban chattel slavery, and by a pretty good margin. It wasn’t for lack of trying by the north’s politicians, but the south had evolved the question of slavery into an existential matter for themselves, and basically said that “without slavery, there is no south”. This made it so that every political attack on slavery was framed as an attack on the very existence of the south itself, which made the subject impossible to negotiate on. There’s a lot more detail that can be got into here, like the insane performative concessions for slavery (that is, in favor of it) that the south demanded that are reincarnated in the braindead performance politics of MAGA today, but that’s a story for another day.

The gist is that when Lincoln, the candidate from the abolitionist party (the republicans. Yeah, a lot has changed lol) won the election, the south had the ultimate shit fit and decided they’d sooner reject the legitimacy of the government than live under a president who, while he probably wouldn’t fully abolish slavery, was against it enough to be part of the abolition party. There was no negotiating this; the founding fathers tried it, and people tried it for decades between then and the war, and when faced with the spectre of maybe having to negotiate some change or moderation, the south looked the north directly in the eye and shit its pants as loud and ferociously as possible. You can see this attitude in the Cornerstone Speech (incidentally, it is a wonderful speech to pull out when someone says the war wasn’t about slavery), which is a speech by the first VP of the Confederacy about how this war is absolutely 100% about preserving slavery.