Because the early rounds of the primaries were in states more keen on Berni and less so on Biden, so Berni was an early front runner.
Some folks have decided to extrapolate that out and assume Berni would have won nationally if only the DNC hadn’t been so strongly opposed to Berni.
Not bullshit. They’re not talking about the heckle they’re talking about the 10 seconds of chanting at 7:18. Watch again. https://youtu.be/2soe8ml_weg?si=uJqnlGGNU4SF8im_