A related dynamic:
Because the shift is happening fast, and because experienced local reporters are used to the status quo, Cuomo/Stevens types get media coverage that is wildly disproportionate to the size of the voters coalitions they represent.
As the Left wins more primaries, there will be more Andrew Cuomo (third party campaigns from the center in deep-blue districts) and Haley Stevens (cynical sabotage in purple states) situations.
Nor were Cuomo and Stevens alone, plenty of institutional actors in the Democratic Party enabled them.