I think this is because every bit of information that points to the real cause of the conflict has them thinking "but...but...but..... We're the good guys! That can't possibly be true! or if it is true, it must mean something different than what it appears to mean!"
To acknowledge the truth, is to tear down their idols, and they cannot bring themselves to do that. They hold Lincoln and company in the same esteem as do the Communists in Russia regarding Lenin and Stalin.
The people they grew up believing to be the good guys, in their minds cannot possibly be the bad guys.
Neither you nor I can the a good guy in regards to the Civil War since the conflict ended 150 years ago and none of us had a damn thing to do with it.
And yet more Lost Cause cockamamie nonsense, nothing more than our pro-Confederates' wet dream.