First can I say that I am not American, therefore my observations are from the outside looking in. America has a 239 years or so history which still has to come to terms with events. You take the date 1776 as the beginning of your country when 13 states rebelled against the British.
As more states joined the Union your country gradually increased until 1860 there were 33 states.
Then 85 years after becoming a nation you nearly destroyed that nation.150 years after the end of the war people in the South still remember their history of course a great deal has to do with nostalgia.
As I said I am not American, but English our history is considerably older. We went through our own civil wars 1641 till 1645 and 1646 till 1649.
However, I want to draw your attention to another even older war (The War of the Roses 1455 till 1487). The war was to see which Royal House would provide our kings The Royal house of York (White Rose) or The Royal house of Lancaster ( Red Rose).
Eventually in 1485 king Richard III (York) was defeated by Henry Tudor(Lancaster)
What does this have to do with America?
Even today there is a huge rivalry between people born either in Lancashire or Yorkshire that war ended 528 years ago.
People have long memories, in the past 50 years the northern states still dominate political life in America, to say that just because there is no segregation anymore, doesn't mean that those in the deep south have to like it.
To display the Confederate flag on number plates etc is their way of showing they still care about their history.
Of course the way you need to come to terms with slavery still needs to be addressed, in Britain we abolished slavery back in 1807.