Possibly because "slavery" was the cause, at least from the Southern side. The motivation for the North was always preservation of the Union.
Whether you wish to acknowledge it or not, the vast bulk of the population believes the war was fought to free the slaves.
And a considerable percentage of Confederacy supporters deny the Southern actions were motivated by slavery - including most of the ones around here. I cannot be responsible for people holding beliefs that are contradicted by all available evidence.
Why do they believe this? Because that is the propaganda that has been spread about it ever since it happened.
Propaganda is not always correct.
Well people have been repeating that assertion for 156 years, so modern people can be forgiven for accepting it as true. I keep pointing out that the Southern States already had slavery, and they had had it for the "four score and seven years" that the nation had existed, and as you yourself have pointed out, it was almost impossible for slavery to be abolished through a constitutional amendment.
So if the Southern states already had it, how was it a cause for them to leave? Were they going to get even more of it by leaving?
To put it differently, what were they going to get by leaving the Union that they didn't already have?
The motivation for the North was always preservation of the Union.
And why would they have a right to "preserve" a Union that was created by breaking with a previous Union? One that lasted far longer than "four score and seven years."
And a considerable percentage of Confederacy supporters deny the Southern actions were motivated by slavery - including most of the ones around here.
Again, did they not already have slavery? How does leaving the Union give them more slavery? Your claim makes no sense. Why would anyone leave a Union because of slavery when that Union allowed slavery? The five Union slave states didn't leave. Did the Union try to take away their slavery?
The accusation that they left the Union to keep something they already had is just nonsense. It is silly on the face of it.
Propaganda is not always correct.
The people who have been spreading it for 156 years, don't care that it's not correct. It justifies what their side did, and so far as they are concerned, that is all that matters. They get to believe in their own minds that their side was the "good guys", and so they would rather believe this than the truth.