Just because it was a key issue doesn't mean it was the only issue. The current "history" books in public schools push it as the only issue.From the second paragraph of the Mississippi articles of secession "Our position is thoroughly identified with the institution of slavery-- the greatest material interest of the world."
Slavery was a complicated issue that covered a lot of ground. But it was a key issue as it related to the southern economy and lifestyle none the less.
https://www.civilwar.org/learn/primary-sources/declaration-causes-seceding-states#Mississippi