The question is worded in a unique manner. In terms of why the people of Hiroshima remained indifferent about the ethics of the bomb, I would suggest that survival of such a cataclysmic event precluded ethical discussion. The victims of such an event are not in a position to engage in full throated ethical discussion. They are focused on survival. Hersey's work depicts individuals who are struggling to survive. They are overcome with the full blown effects of the bomb. Battling through what amounts to being thrown in a pot of scalding oil, where skin "hangs off the body like rags," and offering help to individuals who have been horribly burned, there is little room for ethical discussion. In addition to this, ethical discussion about why the bomb was dropped on them is tantamount to an exercise in insanity. For victims of such an event, in the midst of such an event, to discuss its ethical implications is infinitely regressive. Survival takes precedence in such a condition.
The more appropriate take might be why there was not such a vigorous ethical discussion in the United States. The real question that still lingers is how the United States' policymakers could be so indifferent regarding the ethical implications of dropping the bomb. Given the retreating state of the Japanese and how evidence had already been submitted that the cost of dropping the device could be unprecedented, I would submit that there is some real questioning needed as to why those in the United States who were in the position of power remained in a state of ethical indifference about the dropping of the bomb.