Your latter comment is a good point.

If the SC is first, Clyburn and his party infrastructure can heavily influence the vote there. There is no equivalent in NH (thankfully), which the DNC does not like.

Edit:

The DNC gets to package SC being first as being for equity, while they get to covertly exert influence on the first primary. I would be less distrusting if they picked another state, such as Georgia.

Additionally, the primary calendar just looks like Biden's political infrastructure (wonder why he's pushing for Michigan, ehh?). If Biden has a contested nomination, he'd love to have the calendar in his favor