Master Po has it right as well as the fact that it was fought over States Rights. States Rights overlaps with the slavery issue as that was supposed to be a right left up to the states with the Constitution only limiting the potential importation of slaves in 1808. The fact that only 10% of Southerners owned slaves means the other 90% was fighting only for rich people to own slaves? I would say as value property wise that a slave would be the equivalent to owning a single engined plane nowadays... most of us cannot afford that and the other things we want. So, I do not think most Souterners were actually fighting so the rich could have slaves, it was more the principle, IMO.
The states were much closer in time to the Revolutionary War which, upon its cessasation was settled with 13 peaces between the former 13 colonies, now nation-States on their own only being in a confederation [Articles of Confederation] and Great Britain. The Constitution was sold and ratified with those 13 nations [states] giving up a limited portion of their sovereignty to become a Union slightly stronger than the Confederation, but with enumerated and very limited powers by the national government.
When I was young, a Yankee having been born in Detroit, and living way down in Richmond, Va, I always wondered why the great Robert E Lee did not side with his country but rather decided to stick with his state of Virginia when she moved to secede. After studying history I found that many, perhaps most people, in those times, had a greater allegiance to their state. And the people of the South definitely felt that the national government was going way beyond its reach, that the North was gaining too much power in Congress as the North had the much greater population. And they felt that there was a breach in the contract for limited powers that was promised when ratified.