"...why do they hang on to this extreme southern accent..."
For the same reason people in Baaastan hang onto theirs.
The great thing about living in the South is that people don't worry about how they sound. People are judged mainly by their deeds.
Don't get me started on the people on "Lawn Guyland," Staten Island and SW Brooklyn, who have the most ANNOYING accents in the nation! Its taken me YEARS to shake that.