Okay, so I need to unpack this a bit because there is a lot here. The reason Hillary was historically popular among black people was because she put in the legwork to be seen as such. Bernie did not in 2016, and has shown no evidence of doing anything sense. But even if he did somehow do that, he has bigger problems. His 2016 coalition simply does not exist in 2020. (It was three pronged: true supporters, anti-Clinton and protest voters).
There's a new set of polling of Democratic women that just came out. It's a really interesting read throughout. The polling shows Bernie has some pretty big problems with women voters as well this time. 11% of Democratic women think he would be poor or very poor on women's issues. The only candidate who comes close to those numbers is Bloomberg at 6%. In the full field he is only getting 10% of female democrats votes. He's in 5th behind Biden, Beto, Warren and Harris.) He's running 4th in 18-34 year olds, and 7th among those 65+ at a measly 4% behind Hillary. He's running 4th among white women, Black women and Latina women. 5th among Democrats, and 3rd among Independents. None of Biden, Beto, Harris or Warren supporters name him as their 2nd choice candidate (which would help him when others drop out). In favorable/unfavorable the only candidate who has a worse approval than Bernie among women is Hillary, by a very narrow result.
There are other issues too. 53% of Warren supporters in this poll voted for Bernie in 2016. Nearly 70% of Bernie's supporters in this poll did not vote in 2018, 58% are defined as low awareness voters, and only 52% of supporters in this poll voted for him in 2016.
Put simply, his base is just not there anymore. So he's entering a crowded field where he is in the hole with Black voters and women, the two largest groups of the Democratic party. The fact that he doesn't win Independents is problematic. Put simply, there is no lane to occupy which leads to securing the nomination.