He was in 2016, he's not exactly needed in 2020. I think same is with Biden, but he's the frontrunner, and (I think) large part of his backers back him for two things - for return to normalcy before Trump and just simply by default, like they did with HRC.
Pete, Kamala or even Warren are fresh faces in American politics and they haven't run for president, so some parts of the electorate are backing them, and not Biden or Sanders who are much, much known politicians and they once started for president - the "freshness factor" is out with these two older men, and I think that this factor plays a big role in Democratic primary.
