When you write, “main reason” is sounds like you exclude the possibility there were multiple reasons. In the First American Revolutionary War there were twenty or thirty reasons given by the founding fathers to dissolve the political bands.
The southern states seceded because they became convinced it was in their own best self interest to secede. Or to put it in classic terms, they wanted “to institute new Government, laying its Foundation on such Principles, and organizing its Powers in such Form, as to them shall seem most likely to effect their Safety and Happiness.”
Nothing wrong with that. It was an American tradition.
Slavery. The answer is slavery. The fact that you wouldn’t even use that word in your answer further demonstrates the factual and moral bankruptcy of your arguments.
and protect the institution of slavery. Which they did when they enshrined slavery into their Constitution.