Me thinks you are believing some post-revisionist history.
A nuclear explosive leaves a human-timeframe-defined indelible mark on the planet.
I have heard of no such thing for Japan.
Additionally, the Japanese didn’t have the industrial know how at the time, besides the physics knowledge.
Sciences that did not exist before, outside of nuclear physics, were established, and the technologies based on that science were fully matured within the project scope, many related to the refinement of Uranium and extraction of fissile isotope U235.
Japan had neither the resources, treasure, or industrial knowledge to match that.
At the time, that was the limiter. Most knew it could be done, but no one knew how to make enough material to make it happen for a basic gun style bomb.
More interesting though, if everyone hadn’t been so hot to trot on getting a nuke, it turned out to be much cheaper to use “dirt” (natural uranium metal) in a appropriate manner, and make plutonium.
Plutonium results in smaller devices, and regular old Uranium metal could be turned into Bomb Grade Material.
Even better, Plutonium was separable chemically in relatively simple reactions, further reducing the cost.