oh my sweet summer child, The Democrats have been trying to reduce/eliminate the influence of Iowa and New Hampshire for many years. The reason it's not likely that primaries seasons would be changed solely for a particular candidate is that a party will need to "live with" the result for some time to come, they're not going to be able to rearrange it every election.
South Carolina Democrats, black and white, are much more toward the center of the overall electorate, which is where the national campaigns shift their focus after running left and right in their respective D and R primaries. It is not Biden's specific strength that matters, because it was also the reason "Super Tuesday" was created many years ago to benefit the establishment candidate (Walter Mondale at the time)* https://en.wikipedia.org/wiki/Super_Tuesday#1984:_Beginnings...
And black voters are (or have been) reliably Democrat, which benefits all Democrat candidates, not particular to do with Biden (or Bill Clinton) though they did enjoy outsized black support. Saying the first primary is being moved "to make the electorate look like America" is designed to shut up the Left, who would scream bloody murder if they said "putting this state first is an establishment move because SC is not so left wing" It takes an "establishment move" to change the primaries because the state parties have to change it in the local places with cooperation or opposition from the other party in those states.
* a much younger Biden was a candidate back then too but when, as an English coalminer's great-grandson, Biden's support was centered in the UK so nobody in the US was doing him huge political favors)**
**this is a tongue-in-cheek reference to Biden plagiarizing Niel Kinnock's speech which knocked him out of the 1992 race https://archive.ph/idkaA