I think the organisers were hoping that the warm and wet weather in Japan in July and August (similar to hot and wet weather in Vietnam in the last couple of months) is going to reduce drastically infection. That's also the model that our team @ Imperial College London predicted. The rate of new cases in July/August would drop down to about March levels.
A Chinese study found that CORVID is most infectious when the average outside temperature is 8.7% and humidity around 50%.
A few comments:
But just how are the athletes supposed to train? And coaches travel to athletes, or vice versa.
Many of the organisational staff such as judges are older (having become distinguished within their sports).
Infectivity of coronavirus is, surely, relative not absolute? Might be lower in other conditions but not lower enough to make it safe.
It might prove impossible to complete preparations if Japan is in lockdown.
The airline systems have to reboot themselves to be able to transport that vast numbers around the world.