Read your history. We had Precisly TWO devices, one we were pertty sure would work, one we were not so sure.
Which one do you demo with, and which do you save for the real thing?
Why do people who make supprise and unprovoked attacks on a nation at peace deserve a demonstration? We were already at war - not trying to prevent one.
And one is not enough; it took two for them to surrender. And even then they tried hard to push it.
We actually had enough material produced, it took a long time to produce it, for three bombs. One to test, two to use.
Turned out to be barely enough.