I can't be responsible for what you and the majority of Americans believe in spite of all evidence. Unless, of course, you're saying the South rebelled over slavery. Then you would actually be right on something.
Slavery could not expand within the United States, even if a law to do so had been passed by that Congress which BroJoeK says was under the Control of the Democrats which were committed to slavery.
So what was in it for the South so far as the Corwin Amendment was concerned?
You couldn't set up plantations in the territories because slave intensive cash crops wouldn't grow there.
The majority of slaves never saw the inside of a cotton field.
Expanding it into the Caribbean (it was already there) or Mexico, would have been outside the prerogative of the US to control anyway, so is therefore irrelevant.
But not outside the dreams of the Confederate States.
https://www.gutenberg.org/files/14976/14976-h/14976-h.htm
MOB RULE IN NEW ORLEANS:
ROBERT CHARLES AND HIS FIGHT TO DEATH,
THE STORY OF HIS LIFE,
BURNING HUMAN BEINGS ALIVE,
OTHER LYNCHING STATISTICS
BY
IDA B. WELLS-BARNETT
1900
“New Orleans was the financial center of the Mississippi Valley. From 1835 to 1842 its banking capital exceeded that of New York City, the financial leader of the United States in most years during the antebellum period. The Crescent City’s twenty-six banking companies in 1855 loaned money for the construction of railroads, expansion of plantations, purchase of goods, and many other enterprises.”
I wonder why the CSA didn’t secede from New Orleans?