According to Breitbart, the big difference is a lack of enthusiasm among younger blacks:
Biden’s weakened position, according to the data from Democracy Fund + UCLA Nationscape, stemmed from a lack of enthusiasm for his candidacy among younger black voters. Those surveys indicated that only 57 percent of black voters between the ages of 18 and 29 hold a favorable opinion of the former vice president. On the other hand, 88 percent of African Americans 65-years-of-age and older held a favorable view of Biden.
While I put little stock in the these early polls overall, the consistently stronger performance of Trump among blacks over his first term once again puts Dems on the defensive, seeking to defend their base rather than reaching out to Trump voters.