Warning - the following post contains dangerous levels of generalisation. When looking at things like this, I'd take real care about using monolithic terms like "the South" or "the Union". This discussion really wants to sit at a finer level of granularity than that.
The Lost Cause Myth - it's not a myth, it's an apologia whose context has been lost because it was so successful. And it's not just an apologia for the Confederacy; it's also a whitewash for a Union that doesn't want to keep paying the costs of Reconstruction no matter what happens to black Americans afterwards.
The whole point of the romantic vision of the South was to enable a reconciliation between the white folks who returned to power down South post-Reconstruction, and the other white folks who were in charge up North, who had crushed their armies in the field, burnt their cities, and set occupying troops to control their conquest with martial law. When the US gives up on Reconstruction and the South drifts into the control of men whose wealth, influence, and new-returned power stem from slavery a generation before, there has to be some narrative that allows the losers to pretend not to be greedy evil traitors, and the victors to avoid the guilt of abandoning the reformation of the South to allow greedy evil traitors to reassert the control that hundreds of thousands of Americans died to take away from them.
So the the Lost Cause helps Northerners frame the ACW as being about secession, which was defeated, and not about slavery, which in the South of Jim Crow was... not so much - as long as you don't look too closely under the hood at what actually goes on in Hazzard County besides moonshining and the swaying of Daisy Duke's shorts.
The thing is, with the limited communications media of the next half-century, the myth once established loses its frame of reference; it gets traction at a grassroots level that doesn't question that it's to avoid the guilt of treason (since the grassroots are not the oligarchs who seceded or Redeemed) or of the failure to punish it (since they aren't Northern). They just drink the Kool-Aid of an antebellum Golden Age of chivalric noblesse oblige to grateful Negros who burden their white masters. And who wouldn't want to return to such a fantasy, rather than shoulder the shame of defeat or continuing erosion of wealth and influence that the South experiences as the North sees the Second Industrial Revolution and the American narrative shifts from Manifest Destiny and westward expansion to imperialism (Spanish-American War, Philippines, WW I, Banana Wars and China, etc) both of which leave King Cotton behind?
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