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It was a power move. A way to exploit the rules in their favor.
Congress for the first 50 years of the Union did not say exactly when you had to select your electors, there was a window. South Carolina would postpone their selection until near the end of that window. In a tight race, the combination of selecting near the end of the process and the state legislature controling the selection allowed South Carolina's politicians a greater ability to effect the race. This allowed them to make deals, barter, and generally have an overly representative voice in national politics. At least it's a strategy for such.
The Congress, in 1845, therefore adopted a uniform day on which the States were to choose their Electors. That day - the Tuesday following the first Monday in November in years divisible by four - continues to be the day on which all the States now conduct their presidential elections.