Never mind that the South took the initiative by seceding in defiance of an election of a president by a constitutional majority. Never mind that the Confederacy started the war by firing on the American flag
Democratic leaders had been telling their Irish-American constituents that the wicked Black Republicans were waging the war to free the slaves who would come north and take away the jobs
Slavery was less salient for most Confederate soldiers because it was not controversial. They took slavery for granted as one of the Southern 'rights' and institutions for which they fought, and did not feel compelled to discuss it