I think we are just more "global" -- with TV and the internet, we are exposed to so many different accents that we pick up all sorts of new ways of speaking.  I think, too, that more people move to different areas of the country when it used to be more likely that you stayed closer to "home".  I even imagine, one day, there'll be no more real southern accents. My brother doesn't sound at all southern to me any more -- but he's lived in Arizona for forty years so I guess that's plenty of time to pick up a new accent.