Very simple - No white candidate will win against a credible black candidate in democratic primaries.
When a black candidate is running, black turnout in primaries increases. Black voters made up 25 percent of the Democratic primary electorate in 2016 without a black candidate.
More importantly, they make up a large part of several early southern state voters. Clinton lost whites to Bernie Sanders 48% - 50%, but won black voters by 77% to 22%. She also won the primary.
Any black credible candidate will win blacks by more than 77%, as Obama did. No white candidate will ever win against a credible black candidate again. If Oprah runs, her real competition will be Harris, Booker, or Patrick. I can imagine those candidates will step aside and wait for a VP nod.