What you are asking is something that was advocating by some scientists during the war — that there be a "demonstration."

I have written a bit about this here.

The short version is:

• the US had limited atomic bombs and did not want to risk wasting them

• those in charge of using the bomb thought it was more likely to get their aims (surrender of Japan, show the world what the bomb was) if they just used it on a city (the worse it was, the better)

• they had little sympathy for the Japanese people at that point in the war (not zero, but limited)

• the idea that they would spend billions of dollars on new wonder weapons and not use them was anathema to many of those at the top of the power structure (e.g. General Groves), who set a lot of the agenda

The two nuclear bombs did not end the war and a demonstration would not have either. Japan had endured worse firebombings in Tokyo and other cities. They did not care about civilian losses. They surrendered because the Soviets broke their non agression pact and invaded Manchuria on August 9 1945. That is when they knew it was over.