I'm aware that accents change over time. I'm watching Gone with the Wind for the first time and wondering if the lack of accents is because it's hollywood or because it's a more modern thing.
Depends on the state and who colonized it.
Southern accents in South Carolina or Virginia are British accents slowed down. Most of the people who colonized those areas were from the United Kingdom. They persisted in the South because of the relative isolation of planter communities. You start getting into Tennessee and Western North Carolina, you're looking at Irish and German immigrants so their accents shift accordingly.
That's why whenever you need an actor play someone Southern, you either hire someone from the region or a British person. I thought Gone with the Wind did a decent job of the accents because all of the people are upper-class whites. They would have had a lot of Victorian notions of enunciating and sounding like their British ancestors.