No slavery, no States Rights issue. I used to believe like you, until I read
Alexander Stephens (Confederate VP) speech on the reason for secession. Even though many of the soldiers and military leaders fought for states rights, the political leadership seceded over slavery.
“No slavery, no States Rights issue.”
If the South was fighting for slavery, who was fighting against slavery?
Stevens’ speech is of little or no consequence as it is purely a politician’s speech.
Had they remained in the Union, it was literally impossible to abolish slavery, so your claim doesn't even make sense.