So how the hell did this happen? We all thought it would be somewhere around 60-40, the margin is thirty points wider than anyone thought. How did she swing that many people under everyone's nose? If she'd won by 25-30 we would all have said she overperformed. How is it even possible to surprise by winning by 50? How did none of the polls catch this? How did nobody see this coming?
Honestly this is one of the most impressive political performances I've ever seen. Hillary beat her polling gap by thirty points. When has that ever happened in an election?
The Sanders campaign basically withdrew from South Carolina after losing Nevada. This is what happens when you let your opponent run ads unopposed. Clinton dominated the airwaves in SC, and is doing so all across the south in the run-up to Super Tuesday. That plus the momentum from winning Nevada is what caused her to surge.
None of the polls caught this because nobody bothered polling South Carolina after Nevada, my guess is that wasn't much point to doing it after the Republican primaries since everyone knew Clinton would win. The Clemson poll was basically the only post-NV South Carolina poll we got, and it ended up being quite accurate.