Lack of vision, I guess. Foolishness. A determination to have done with the North, before the North, swelling to unguessable dimensions with immigrant trolls, eventually bore down the South and crushed it despite anything Northern Democrats could do.
Possibly, the Southerners correctly read the political tea leaves, that the Republican and Abolitionist propaganda of sectional demonization had worked, and the North was now united as a politial monolith against the South. Under such circumstances, as Robert Rhett's circular appeal to the Southern States to join the secessionist movement and Robert Toombs's speech to the Georgia secession commission to the same end show, many Southerners felt that they could no longer save themselves from oppression by the North.
Or could it be that the Republican platform supported the ridiculous (to the southerners) idea that 'all men are created equal'? Or perhaps it was the plank that said it was the rights of states to control their own institutions? Or the plank against the expansion of slavery or the one against the reintroduction of slave trading? Or perhaps it was the lack of a plank advocating the conquest of Cuba? I can't imagine southern Democrats signing on to any of that.