Obama has absolutely absurd charisma. He's the Democrat version of Trump - knows exactly what to say to his base and knows how to convince moderates he's not insane.

Clinton and Biden have the charisma of a limp noodle. Sanders has absurd charisma, but he's seen as too big of a threat to Democrat lobbyists and big corporations.

Sanders would've mopped the floor with Trump because he would've actually been able to grab the 18-44 demographic (which last saw peaks in 1992 Clinton/Gore and 2008 Obama/Biden, both to unseat a Republican and, coincidentally, a Bush).

Sanders would have been able to avoid the collapse in turnout from working-class Black people in 2016.

Sanders would've stopped the increasing right-wing radicalization of the youth of America, or provided a counterweight for left-wing economic radicalization.

The US federal elections are basically a pony show and the DNC doesn't know how to play the game without throwing out their playbook.