First off in many counties in SC most people simply cannot or will not pay for broadband. I worked with a guy when I was younger who shared a phone with a neighbor. They're whole neighborhood was shacks.
Are you sure it wasn't just BellSouth refusing to upgrade the party line?
Good question. I assume it was for money reasons but there is a chance your right. I can tell you I was unable to get high speed till 2000. Last year or maybe the year before my 1 meg download was upgraded to a 3 meg download. That was the top tier till 2 months ago when I received a letter saying they were adding higher teirs if it's avaible in your neighborhood.