I personally believe that Iowa and New Hampshire are essentially meaningless. These states are 90+% white and do not represent the demographics of the party. The primary process does not really start until South Carolina where the voting population reflects the demographics of the party as a whole.
I found this article from 538 to be very informative
https://fivethirtyeight.com/features/why-do-black-democrats-usually-prefer-establishment-candidates/
So here are a few explanations for why black voters have tended to side with the establishment wing of the Democratic Party. I have tried to order these explanations from strongest to weakest (in my view, at least):
1. Establishment candidates typically have existing ties to the black community
This will sound tautological, but an establishment candidate is … well … established. A candidate who is part of the establishment wing of the Democratic Party likely has fairly strong ties to major constituencies in the party, such as labor unions, women’s rights groups and, of course, black leaders and voters. So when black voters backed Gov. Andrew Cuomo over Cynthia Nixon in New York’s Democratic gubernatorial primary last year, or Andy Beshear over Adam Edelen in Kentucky’s Democratic gubernatorial primary earlier this year, that was not shocking. Not only did Beshear and Cuomo spend years developing their own ties with the black communities in their states, but their fathers did, too. (Steve Beshear was governor of Kentucky, Mario Cuomo the governor of New York.)
Clinton in 2016 and Biden in 2020 similarly entered the primaries with longstanding ties to black voters. It’s worth considering if the story here is not that establishment candidates are smarter in appealing and connecting with black voters during the campaign, compared to anti-establishment candidates. Maybe it’s that the establishment candidate in a race is likely to be the person who enters the campaign with the strongest support among black voters.
2. Black voters are pragmatic
White Democrats are significantly more likely than black Democrats to describe themselves as liberal. Perhaps that’s the simple explanation for why most black voters eschew more liberal candidates. But scholars of black voters argue that the liberal-moderate-conservative framework does not apply well to predicting the actual policy positions and voting behavior of black Americans.
I agree with the conclusion of this article
But even if Sanders or Warren gets more support among black voters in 2020 than the Vermont senator did in 2016, I tend to think Biden will remain fairly popular with black voters overall — because of his ties to Obama and other black leaders and the perception that he can defeat Trump. So there is a very real possibility that black voters will play the same role in the 2020 presidential primary that they have played in Democratic politics over much of the last four years: blocking the path of the liberal left as it attempts to dethrone the party’s establishment.