She's a politician, and she's had a taste of power. It will be very difficult for her to give up that power. She'll do ANYTHING to hold on to it, believing, as they all do, that power is better off in her hands, than in someone else's'.
My hope is that she's listened carefully to Bernie and learned from him. He's one of the very few, perhaps the only one in Congress, who stayed out of the trap.
As far as letting her membership lapse, who knows? My guess is she was biding her time, seeing which way the wind blows. Voters are warming up to socialists at the moment, and she sees a chance to make that party work for her, so she reupped.