Here's why I think that is.
1) Voter suppression is going to be as bad, or worse, as it was in 2016. Republicans and Russians alike are taking every measure they can to ensure black voters stay home.
2) Bigots everywhere are going to be EXTRA energized, as Trump has delivered on the front of attacking minorities. He was successful in many areas, such as curbing black and brown immigrants and at setting back the rights of minorities. The bigots are more energized than they've been in a while.
3) People will stay home or vote third party if Bernie or Warren aren't the nom. Which they won't be. It will be Biden, the most like Hillary. Dems have learned nothing and are repeating the same errors. People are desperate and they aren't going to respond well to the same ol same ol' like Biden or Buttigieg.
4) The white people who voted for Trump, and yet were screwed over by him STILL worship him. Look at the farmers. Ties into #2 on my list.