He let them do it. Put up no protest and then gave them his support after they fought him with their whole apparatus.

He's also a Zionist, weak tea when it comes to policy, and completely incapable as an organizer, throwing away his network twice after losing. He was always a compromise candidate at best from a left perspective and is only perceived as particularly left wing in comparison to the duopoly status quo of proud neoliberal racist vs. false pretense neoliberal racist.

He was never gonna win, the DNC has a party sheepdog every election cycle to keep disenfranchised voters rounded up in the party with the illusion that very soon, if they stick around, they too will have a seat at their table. By the time voters realized they got fucked over, again, it's too late to form outside coalitions.