Christine O'Donnell and Alvin Greene

[Smash255]

Pretty much the reasons stated above. 

South Carolina was never up for grabs, Delaware on the other hand was, and O"Donnell knocking off Castle completely changed the dimension of that race.  That was not an issue in S.C.  Even if Rawls won the Primary, the dimension of the race (that being Demint having it locked down) wouldn't have changed. 

Also the influence of the tea party and Palin were big stories this year.  The tea party and Palin had nothing to do with Greene getting the nomination in South Carolina.  They had everything to do with O'Donnell knocking off Castle in Delaware, resulting in the GOP getting stomped in the state.

You also have the fact that in addition to Palin, prominent Republicans were supporting O'Donnell, you didn't have that in S.C with Greene.  Also of course the millions upon millions of $$$ compared to basically nothing.