It was about more - you’re right about that. However, slavery was a big factor, as Federal government pressure to end slavery was one of the things that the Confederate states were resisting.
While the Republican party had a growing Abolitionist element, the party platform itself, which Lincoln supported, was only to prevent the expansion of slavery into the territories, not to abolish it. People in the South naturally considered this restriction a step on the road to Abolition. After all, the British Empire didn't abolish slavery all at once either.
It was going to come down eventually to either two nations, one slave and one free, or no slavery at all. The war just forced the decision. Sometimes, that's just what it takes. Reminds me of the Protest Warriors' banner "War Has Never Solved Anything (except for ending slavery, Fascism, Nazism, and Communism!)
But, as some have observed, every solution breeds new problems; so it was with this one. The federal government emerged from the conflict with new power which it gradually began using over the next several generations to give us the world we live in today, a world where a gaggle of liberals in DC can tell us how much water our toilets can flush because somebody at the dinner table brought a fork across the state line.
People who wave the Confederate flag are reminding us of the last time the states stood up against the federal government. The one thing they certainly are not doing is agitating to bring back chattel slavery; rather, they are protesting against the slavery that now exists.