Biden was the favorite in Democratic primary polling wire to wire:
en.wikipedia.org
He was also the favorite to win the general election wire to wire:
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projects.fivethirtyeight.com
Bernie lost the last two Democratic primaries and it was not particularly close. We aren't talking about a crowded field where he didn't get traction, we're talking about where it was a clear choice between him and another Democrat and he lost badly both times. It's not going to happen and wouldn't happen even if he ran again.