Well, according to Mark Hughes and others, Ferrari and Alfa Romeo were going into that teams meeting determined not to go racing, and Kimi had already said he thought racing wasn't a good idea, while Seb' also had aired 'doubts' (ie. polite, sticking to FIA line, but still saying it isn't a good idea IMO); Those two leaving makes clear sense
Lewis Hamilton had already made clear he wasn't happy about racing, and I suppose while Toto got into the meeting feeling they could race, their company HQ shared (with mood in Germany, Bavaria now closing schools etc. down and others closing more and more, no real surprise) Hamilton's feeling, so no surprise he was gone as well.
We haven't heard about the other drivers (suppose Norris, Sainz Jr. may have some forms to deal with perhaps though?), but why stick around until F1/FOM,FIA, organisers and the local government played chicken with who could wait the longest to cancel?
As for other GP's, well, McLaren won't be able make it at least to Bahrain in a week, and Vietnam is why Chase Carey was in the air rather than in AUS being decisive (I would make that his defence, at least); let's see what he discussed, but Hughes and Joe Saward at least suggest that one to be 'postponed' too, and expectations to only resume racing in June.