Why did South Carolina continue to select their Presidential Electors by legislative choice until after the Civil War?
What were the reasons for South Carolina to not use a popular vote to decide the Electors for so long compared to other states and why did they finally decide to put the decision to a popular vote in future elections after the Civil War?
Let's remember here that South Carolina was, until the Civil War ended, a majority-black state with an all-white government and a slave-based economy. It was, therefore, intensely anti-democratic in its governance. This helped motivate its refusal to let the popular vote select electors. Up through 1845, states didn't all select their electors at the same time; there was a window of a couple of weeks during which slates could be announced. South Carolina deliberately held back its selection to the end of the window, so that they could see how the other voting slates were doing, and negotiate with political parties and other states before its legislators made their decision. It could only do that because its legislators, not its voters, were making the decision.
After an 1845 reform, all elector slates were announced the same day. I suppose South Carolina held out as the only state with legislators picking the electors out of inertia, habit, and legislators not wanting to give up a special power they were used to, even though most of the advantage was gone.
South Carolina switched to popular vote after the Civil War because of Reconstruction. Suddenly U.S. troops were enforcing the rights of the black majority to cast ballots; suddenly its governors and legislators and senators were black, reflecting the actual will of the people. The Reconstruction government imposed the popular selection of electors, bringing South Carolina in line with the rest of the states.
When Reconstruction ended and blacks were again deprived of the vote, South Carolina didn't, on that issue, change back.