Because Biden is a more centrist candidate than some of the others. The assumption, perhaps inaccurate, is that rust belt voters who abandoned the Democrats in 2016 are more centrist and so would respond better to a centirst than a more leftist candidate.
But we also know that, in general, the people who supported Trump weren't suffering from economic anxiety. In fact, voters who labeled the economy as their top issue voted mostly for Clinton. Instead, Trump voters were mobilized by social anxiety and fear of a racially and culturally changing US. In that case, Biden, who is a white man, may fare better than some of the women or people of color who are running. But, it may also be that these voters are simply no longer compatible with a Democratic party which is increasingly devoted to racial tolerance.