I am surprised that nobody considered postponing a couple of months until late September/early October. Nobody seemed to seriously entertain this idea. This would give time for the epidemic to die down without having put things on hold for a whole year. Perhaps that would still be disruptive to some athletes training. Incidentally, the '88 Games were also held late around that time of year instead of the summer (for reasons not clear to me) and that was a success.
NBC paid $1.45B for the rights to broadcast the Olympics. As a midsummer event, the Olympics only have regular season baseball to compete for attention. Push the Olympics into the fall and it has to compete with the NFL and MLB playoffs. If the Olympics were pushed to the fall, I'm sure NBC would insist, and the IOC would have to pay back a good chunk of that $1.45B. I believe the Seoul Olympics and the first Tokyo Olympics were held in October to avoid summer weather. Doesn't explain why this Tokyo Olympics was scheduled in its usual midsummer slot. Probably again because of money.