I would guess size plays a factor. This country is large enough that if you really wanted to, you could live out your whole life comfortably without ever leaving the South (and indeed, most do down here). Without a mixing of ideas, people just get feedback over and over again. If you mention Democrats down here, you get an instant "those baby killers?" response. It's not even deliberate anymore, just a reflex.
Also, more people post-Civil War should've cracked down on this shit faster, but if we're being honest, most of the Union didn't really care about slavery either. They just wanted the war to end quickly and move on.
To piggy back another question onto yours about civil wars in other countries, how many other countries have a civil war, then see one of their rebelling regions come back but use the battle flag of treason as their state flag? Here in MS, we don't even blink when we see the damn thing on every building and bumper sticker.