I forgot, every movie has to show every aspect of every situation it uses.
There was a reason the movie was called Oppenheimr, and not Hiroshima, and why the overarching theme was about the political fallout of his career. It was a bio-pic about Oppenheimer. The reason it didn't go into detail about the literal fallout of the bombs is because that's not what the movie was about. There's plenty of documentaries and movies that are about that. If that's what you want, go watch them.
Or make (and advertise) them, like Cameron.