Party politics got a late start here. We didn't start making any elections officially partisan until the 1850's. Our weak executive branch made the case for a statewide party weak. Then the Civil War and Reconstruction caused our politics to be polarized by the issues of race and sectionalism, so that once Tillman caused the blacks and Yankees to be disenfranchised in the 1890's, the voters who were left were monolithically Democratic. Our election laws still carry remnants from when the Democrats did their best to hamper the Republicans. For instance, I think we may be the only State that requires the parties to pay the cost of holding primaries. I don't think the Republicans held a primary in South Carolina until the 1970's as a result, and used county conventions instead as they were cheaper, tho they were far less effective in helping to get out the vote.
I think he means why is South Carolina significantly (about 10%) more Democratic than other states than Georgia, Alabama, Louisana, etc.
