'Make America Great Again' doesn't work if those halcyon days included slavery (or enforced segregation, or redlining, or any of the other structural racisms that the early and mid US incorporated.)
This make them feel - not bad, exactly, but like they cannot be as proud of their history as they otherwise could be. These studies of history make many white people feel bad, because it reminds them that a significant part of this country came from the labor of slaves. We are seeing the same thing happen today with the attempted cancellation of both CRT and the 1619 project. They could instead take pride in their ancestors who stood up to an evil north, rather than trying to reconcile their heritage of slavery with a more modern view of civil rights. By reframing the Civil War as a 'war of northen aggression' having nothing to do with slavery, whites in the south could avoid any residual feelings of shame associated with the position they took on slavery.
