Just because the North didn't fight the war to end slavery doesn't mean that the south didn't launch their rebellion to protect it.
The South went to war to end tyrannical central government which was a violation of the contract between the people and the governed.
That government wasn't so tyrannical when the southern leadership dominated it. All of a sudden it became an unbearable burden when it looked like it would limit the expansion of slavery. And yet you claim the two are not related.
Slavery was very much one of the reasons the South went to war. It was never one of the reasons the North went to war. Lincoln wanted his union and was willing to do anything to preserve it including fighting a war against his countrymen and guaranteeing slavery permanently.