Was trying to answer this question for a school age child writing an essay about Japan. Short of Wikipedia's "hundreds of years ago, you arranged for fresher fish by transporting it live" nothing meaningful popped up in my searches - and that doesn't offer explanation because of course you can cook fish after you transport it live.
So, is there some reason why eating raw fish became so popular/prevalent in Japan's seaward areas specifically (compared to other sea-adjacent areas of other nations)? Was it some specific quality of fish native to Japanese sea waters? Or just an accident of culture?