Rule 12 Timing of the Delegate Selection Process 

  No meetings, caucuses, conventions or primaries which constitute the first determining stage in the presidential nomination process (the date of the primary in primary states, and the date of the first tier caucus in caucus states) may be held prior to the first Tuesday in March or after the second Tuesday in June in the calendar year of the national convention. Provided, however, that the Iowa precinct caucuses may be held no earlier than 29 days before the first Tuesday in March; that the New Hampshire primary may be held no earlier than 21 days before the first Tuesday in March; that the Nevada first-tier caucuses may be held no earlier than 10 days before the first Tuesday in March; and that the South Carolina primary may be held no earlier than 3 days before the first Tuesday in March.

This is what created "Super Tuesday", which is evidence of a race to the bottom that has gone as far down as it can go. It also shows that 4 states were carved out probably as favors (does anyone know more here?). In order to change this, probably a bunch of people who are being treated favorably now will get upset at losing their privilege, and no one in leadership wants to crack that egg. Seeing as that South Carolina, one of the favored states anointed to hold a primary before before Super Tuesday, played such a decisive role in the nomination of Joe Biden, I think the practice of keeping these states first is going to continue for the foreseeable future.

Jim Clyburn of S. Carolina is widely credited of resuscitating Joe Biden's candidacy after his sad start in IA, NH, and NV. He placed a distant 4th in IA, 5th in NH, and Bernie blew out NV with 40% of the vote. At that point, Clyburn endorsed Biden, and Biden ended up winning SC. Then a bunch of candidates dropped out of the race before Super Tuesday (some who were doing much better than Biden overall) and support coalesced behind Biden.

If anything, the slow roll of IA, NH, SC and NV allows the party to calibrate the primary process. If things start going a way they don't like, they can make quick changes like asking a bunch of candidates to drop out of the race (I'm not saying Bernie would have won had they not done this, I'm just pointing out it was obviously done, and what enabled them to do this was the fact that all primaries are not held on the same day).