Either Biden made backroom deals, or the party did, or Amy and Pete's big donors told them point blank the gravy train was stopping, or something along those lines. Even if they did it fully spontaneously, that is them using the positions of power and influence they have obtained to try to line the party up behind the guy who challenges absolutely nothing major in the status quo! The guy with a pretty bad record of reactionary legislation and whose brain is currently the consistency of oatmeal! To the extent that the Dems present themselves as a party of meritocratic openness, it utterly undermines their credibility and message and broadcasts loud and clear where their real allegiances are.