You don't know the (binding!) DNC rules? He had the delegates committed to him and couldn't have been forced out. And he didn't want to quit. It was a pressure campaign to make him look out of control, and he's always been better at negotiating behind closed doors than he is at PR so he couldn't fight back effectively. Pretty much a chicken race to see who backs down first.
From the timing of everything going on I suspect that with the combination of the media campaign, backstabbing in the party, him being sick x2 and tired, and finally a Republican convention and VP pick being newsworthily weird (and probably also with some commitments from others to back Kamala if he stepped down) I bet he thought there wouldn't be any better opportunity to do it.
(I was expecting the switch to end in chaos, somehow for once DNC managed to do something coordinated, DESPITE many donors also wanting to ditch Kamala)
I certainly hope Kamala will be harsher against Netanyahu and back the ICC rulings