Bernie has been subjected to a thousand postmortems so it's probably worth asking what everyone else in the field did wrong (answer in almost every case is not directly challenging Biden on electability).

There were plenty of candidates who could've argued, "if you think Joe Biden is electable consider how much better it would be to nominate a moderate who isn't 78 years-old and doesn't have four-decade senate record for opponents to pick over" but they didn't try.

We happen to have slid into a pandemic where a front porch style campaign is optimal, which is a good break for Biden but obviously wasn't foreseeable during the critical months of the campaign.