Anything less was less likely to succeed in ending the war, so it was used in the most effective way possible...unfortunately.
At the point at which the atomic bombs were dropped, Japan was seeking to surrender and had been communicating this to the White House (via Russia, earlier in 1945). Their big problem with the Potsdam Declaration was that it seemed to indicate that the Emperor could/would be forced to step down and be tried as a war criminal, which was unthinkable to them because the Emperor was considered
arahitogami, divinity incarnate. It would be like asking the US to hand over Jesus after a second coming.
Hirohito remained in power, and was not tried as a war criminal. The conditions the Allies would not agree to before the bombing were exactly what happened after the bombing. Hmm.