Hiroshima was at that point still completely unharmed, had no prisons for captured American soldiers and was roughly the same size as German cities that were previously bombed, so they had an easy way to compare the effectiveness of the bombs to classic bombing campaigns. Nagasaki was just a fallback target because Kokura was too cloudy (which Nagasaki was then as well, but they didn’t have enough fuel to return with the bomb on board, so they decided to drop it anyway).
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Hiroshima was at that point still completely unharmed, had no prisons for captured American soldiers and was roughly the same size as German cities that were previously bombed, so they had an easy way to compare the effectiveness of the bombs to classic bombing campaigns. Nagasaki was just a fallback target because Kokura was too cloudy (which Nagasaki was then as well, but they didn’t have enough fuel to return with the bomb on board, so they decided to drop it anyway).