People were angry about Pearl Harbour.
People were tired of war.
The targets were chosen (in part) from undamaged cities so they could measure the effects of the devices in an urban environment.
Look here for some interesting history / physics / engineering
https://en.wikipedia.org/wiki/Little_Boy
>After being selected in April 1945, Hiroshima was spared conventional bombing to serve as a pristine target, where the effects of a nuclear bomb on an undamaged city could be observed. While damage could be studied later, the energy yield of the untested Little Boy design could be determined only at the moment of detonation, using instruments dropped by parachute from a plane flying in formation with the one that dropped the bomb. Radio-transmitted data from these instruments indicated a yield of about 15 kilotons.
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