I think that by including the Japanese perspective, the film would have lost its focus on Oppenheimer himself. Considering that the film is entirely from his perspective, it wouldn't have made sense to me to cut away to Japan. This film is more so about how America perceived Oppenheimer than anything else, I feel. The Japanese perspective on these events are best left to another film, even if it feels as though something is missing. That fragmentation - the lack of that secondary perspective - is very much intentional, and I like to think that most people seeing this film know of the atrocities that happened because of the atomic bomb.

To show the Japanese perspective here might have been to sensationalise the bombings of Hiroshima and Nagasaki, which would have been far worse.