South Carolina is Winner-Take-All by congressional district plus statewide. Trump won every congressional district and the state overall. This gave him every delegate. South Carolina is the only early state allowed to award delegates non-proportionally. This is a holdover from before the rule was instituted.
South Carolina has seven congressional districts which get three delegates each and the remaining twenty-nine delegates are awarded based on the statewide vote. This often results in multiple candidates getting delegates, as more than one candidate will win a congressional district. But in this case, Trump's support was both broad enough and deep enough to win every congressional district.
Other states with a similar system award two delegates to the top vote getter in each congressional district and one to the second best. Unless a candidate beats the threshold (usually 50%) or the second best fails to meet a minimum threshold. This is why Alabama only awarded two delegates to Cruz and none to the other non-Trump candidates. But South Carolina has no threshold. It can be viewed as eight elections that are each winner-take-all.