Because Lincoln was resolutely opposed to the expansion of slavery in the West, which was the real casus belli for the South.

For example, Mississippi produced "A Declaration of the Immediate Causes which Induce and Justify the Secession of the State of Mississippi from the Federal Union." Notable passages include:

“Our position is thoroughly identified with the institution of slavery-- the greatest material interest of the world.”

“It [the federal government] refuses the admission of new slave States into the Union, and seeks to extinguish it by confining it within its present limits, denying the power of expansion.”

“Utter subjugation awaits us in the Union, if we should consent longer to remain in it.”

“We must either submit to degradation, and to the loss of property worth four billions of money, or we must secede from the Union framed by our fathers, to secure this as well as every other species of property.”

The second of these, in particular, indicates that the South (or at least Mississippi political elites) had come to see the expansion of slavery as necessary for its survival. Ergo, the war was triggered by limits to the expansion, rather than an immediate threat to the existence, of slavery.

can someone explain what the Corwin amendment was? I am drawing a blank.