The quote:
“Given Joe Biden’s incredible record, given Donald Trump’s terrible record: he should be mopping the floor with Donald Trump. Joe Biden is running against a criminal. It should not be even close. And there is only one reason it is close. And that is the president’s age.”
My pet theory is that the establishment wing of the party (that largely controls the DNC) wants to have a specific coalition that keeps them in power within the party. Like, if Democrats wanted to be the party of the working class or appeal to rural voters, they could but would require leadership that isn’t from New York, San Francisco, or other similarly rich places.
So, under the leadership of Clinton, Pelosi, and Schumer, they chose to make the swing voters the ones they appeal to most. Maybe Bernie’s positive populism would have matched up better against Trump’s negative populism than Clinton’s outdated neoliberalism. But leadership and the DNC would rather lose an election and keep control than win but lose their place atop the party hierarchy.
I think it’s simultaneously less conspiratorial and more nefarious than that: the establishment wing of the party likes power and power means control over campaign funding, so they pander to large corporate donors by suppressing anti-corporate populists.