That's certinly part of it.If Bernie campaigned poorly, Biden campaigned catastrophically bad. He had one campaign office in California and was barely present (if he was present at all) in most of the states he won on Super Tuesday. He barely spent any money. He barely had a grassroots or a ground game.
The issue with Bernie's campaign was, as someone said earlier, that he appeared to be running against the Democrats rather than with the Democrats, especially to the black community. Whenever Bernie railed against "the establishment", people saw him railing against those very same people that have pushed for civil rights and equality, which obviously wasn't the case, but a vague term like "establishment" tends to group a lot of people together.
Bernie could have presented himself as the natural follow-up to FDR, JFK, Johnson, and Obama. That he was pushing the same dream and the same policies they did to the natural next step. That the Democratic Party is the institution that has fought for the rights of minorities, the rights of workers, and he will take that fight to the next level.
But, he didn't. He rallied against the body that for so long has been the only real hope of these groups. He even left the party between 2016 and 2020.
That just doesn't make sense for someone running to lead the Democratic Party, and it hurt him.