It's rather difficult to answer everyone, so I'll just lay my thoughts out here then:
I feel the answer is a sociological one. A sociologist can analyze why social movements have changed since the 1960s, from militant-on-the-street activists to liberals mailing money to NGOs to do the activism by proxy or through the judicial system.
That activism by proxy is what the Sanders campaign appeared to be. Not only a "push Clinton left" campaign, but one where canvassing in a safe, legal, and calm manner for electoralism will appoint our delegate to enact changee on our behalf. Our delegate had concrete demands, an agenda, and public display of grievance with a reputation willing to address them and resolve them.
Bernie's failure scattered activists to racial justice, anti-police violence activism, labor activism, healthcare activism, wage activism, etc. Divorced from a focal point, repressed through state violence, denied a pool of malleable and sympathetic liberals with a Biden vicotry, political agitation and prospect to challenge power has become diffused.
That diffusion came with a Biden victory and I blame Sanders for running in 2020 to begin with. Sanders had the bully pulpit to concentrate power and popular will and showcase to Americans: They're letting you die/leading you to your death for profit (via Covid-19.
It might be an indulgence in Great Man theory. But the campaign itself was a focal point to create parallel institutions outside of the Democrats that were not riddled with careerists.
Idk, missed opportunity is what I think.