Sorry I didn't look to see what that route was served with every day, I just picked 2 relatively close cities, but I can say that half the flights on that route used to be rjs as I used to fly that route for my last job regularly. I also know that up until about 4 months ago bna was served heavily by rjs but demand was way to much for rjsBad example. There are 11 flights a day between both SAV-ATL and BNA-ATL. All of them are mainline flights.
The places that Delta serves with RJ flights usually only have 4-5 flights a day to begin with. Those people are more likely to drive from the closer places because there is less frequency and longer layovers in ATL. Something like Chattanooga TN to Columbus GA. And even CHA has mainline service. So does AGS and GSP. It's a 20 minute flight to those places.