I think because they want a state whose Democratic electorate has a high percentage of Black voters. In 2016 the % of Dem primary voters in SC who were Black was 58%. (Only AL and MS were higher.) Of course, that also makes it one of the least-representative Dem electorates in the country.
We Re-Ordered The Entire Democratic Primary Calendar To Better Represent The Party’s VotersFew things stay the same from one presidential primary season to another, but one remains constant: Iowa holds its caucuses first, and New Hampshire follows wit…web.archive.org