With a little help from his friends.

Bernie was still generating a lot of enthusiasm as evidenced by his many many many small donations and fundraising ability.

Bernie lost because the establishment rallied around Biden

“Sanders won not only with voters under 30 and people who identify as very liberal, but also with men, women, Hispanics (overwhelmingly), voters 45 to 64 and people with and without college degrees, according to polls conducted as voters entered caucus sites. He also did well among black voters and moderates, finishing a close second to Joe Biden with both groups, which are supposed to be the former vice president's base.”

https://share.google/HHMKRnAOj8Ld5G6VU

Sander's best shot at winning the presidential nomination was 2016, not 2020.  He had a one-on-one matchup with Hillary, whereas the field was more crowded at the start of the 2020 campaign. 

Sanders lost to Biden for two reasons. He wasn't able to generate the same level of enthusiasm and support among youth voters in 2020.  Biden also did well with Black, suburban and working class voters.
https://www.pbs.org/newshour/politics/sanders-banked-on-young-voters-heres-how-the-numbers-have-played-out

Finally, Sanders and his supporters honked off many older progressives like me with their boorish behavior at the Democratic convention in 2016.  I would have voted for him if he had won the nomination in 2020, but not in primary.  I had him ranked just above Tulsi.

Yet another L for the Nixon Democrat.