But not to be tedious in enumerating the numerous changes for the better, allow me to allude to one other -- though last, not least. The new constitution has put at rest, forever, all the agitating questions relating to our peculiar institution -- African slavery as it exists amongst us -- the proper status of the negro in our form of civilization. This was the immediate cause of the late rupture and present revolution. Jefferson in his forecast, had anticipated this, as the "rock upon which the old Union would split." He was right. What was conjecture with him, is now a realized fact. But whether he fully comprehended the great truth upon which that rock stood and stands, may be doubted. The prevailing ideas entertained by him and most of the leading statesmen at the time of the formation of the old constitution, were that the enslavement of the African was in violation of the laws of nature; that it was wrong in principle, socially, morally, and politically. It was an evil they knew not well how to deal with, but the general opinion of the men of that day was that, somehow or other in the order of Providence, the institution would be evanescent and pass away. This idea, though not incorporated in the constitution, was the prevailing idea at that time. The constitution, it is true, secured every essential guarantee to the institution while it should last, and hence no argument can be justly urged against the constitutional guarantees thus secured, because of the common sentiment of the day. Those ideas, however, were fundamentally wrong. They rested upon the assumption of the equality of races. This was an error. It was a sandy foundation, and the government built upon it fell when the "storm came and the wind blew."
Now, what part of immediate cause do you not understand?
To reply to your post, "immediate cause" I understand just fine, but it isn't the same thing as "main cause", "root cause", "underlying cause", or anything like that. Yes, the slavery issue had been pushed by Republican and Abolitionist propagandists until everyone was acting like a Jerry Springer audience (Jerry Springer is evil, bring me the head of Jerry Springer), and they were all ready to start killing each other.
But here is the thing. Control of agenda, control of one's own political destiny, is far more potent a first cause and source of motive than any individual issue, because control goes to ALL issues and so is as powerfully motivating as the sum of all issues in play.
Control, rather than any one issue, even a big one like the basis of the Southern economy, is the really big motive in secession, and if you would take the time to review Stephens's other speeches such as the one I linked to in my post to x above, you would see that control links like a fetter all the issues he mentions either in the Cornerstone Speech or his speech to the Georgia legislature a few months earlier .
You can't raise a control issue like the one in play with Lincoln's election, without simultaneously setting into motion all the other causes and issues that pressed upon the actors of that day.