For the same reason they didn’t firebomb a forest to cinders before [doing that to Tokyo](https://en.wikipedia.org/wiki/Bombing_of_Tokyo_(10_March_1945)) five months before the first atomic bombing.
The US Army Air Forces spent the spring and summer of 1945 systematically burning to the ground every Japanese city of even middling significance within their range. And they finished the job, too, sparing only a few cities reserved as atomic bomb targets to allow the expected new weapon’s destructive power to be accurately assessed. The idea was that this rain of destruction would both disrupt the Japanese economy and demoralize the population, hopefully winning the war from the air or at least making invasion significantly easier.
While everyone recognized the atomic bombs were a revolutionary weapon technologically, they did not change the moral calculus of the ongoing city destruction campaign one bit.
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