I'm not sure, maybe someone living in Japan can better comment on this. I don't think the Wuhan virus has hit Japan all that hard as they have relatively low amount of reported cases, assuming they're reporting their cases accurately. There's been a state of emergency declared there, but it's not enforced quarantine like in western countries.Maybe it's a cultural thing like they can't come together and actually organize this thing unless it's in person, with the same applying to game development in some level.
Who knows, maybe they really need more time for developing more titles after all. Although it seems Breath of the Wild 2 won't be releasing this year.