Nagasaki wasn't even the selected target for that day it was Kitakushu. However the cloud cover over Kitakushu was too heavy to determine the target so Nagasak

Nagasaki wasn't even the selected target for that day it was Kitakushu. However the cloud cover over Kitakushu was too heavy to determine the target so Nagasaki was very much a secondary target and they dropped it an area of low population density in a valley that would minimize the fatalities while still making the intended point to the Emperor. It really just had to emphasize that Hiroshima wasn't a fluke or a meteor but a new kind of weapon and the US had more than one of them.