Because conservatives see the Confederacy as the last attempt of the states to resist the all-powerful federal juggernaut. We aren't allowed to consider the war as about anything but slavery, but in fact it was about who got to decide the question (and, as it worked out, all subsequent questions.)
Do I really need to put on my own flameproof shorts now...?
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.