Just as some people are famous for being famous, so some places become visit-worthy because they're well visited. The reason that Hiroshima is on so many tourists' itinerary is because outside of Japan's top five most populous cities **, Hiroshima and Nagasaki are probably the only two that EVERYONE has heard of, and Hiroshima is easier to get to. Clearly Hiroshima's fame is largely a result of its history as the first city to have been destroyed by an atomic bomb.

The city has few pre-war historical buildings or neighborhoods of the kind that tourists flock to in other places ... Only the castle ruins, and a few buildings that are famous for having survived the bombing. For that reason, the typical tourist visit takes in the various peace memorials, some time in the park, and then on to Miyajima. That said, it's a very pleasant city with an unusual amount of green space and wide avenues. It's a nice place to cycle around and the people are welcoming. I feel that it has a more outward-looking / international feeling than most places in Japan. Whether that's because of its history as a trading port or its status as a peace city, I can't say for sure. Probably the latter.

EDIT - It also plays host to a greater number of international conferences than similarly sized cities, including trade conferences and academic / scientific / medical forums etc. . Again, that probably stems from its status as a city that has set itself up to promote global peace. All of this creates a virtuous cycle that makes it visit-worthy.

FInally, perhaps we should look at the visitors rather than the city to understand why some places are more visited than others. I think a lot of people go to the same four or five places as everyone else because they want to be able to tell the folks back home that they went to those four or five places. It's tempting to say that this is because of social media but I think it predates that.

** Or let's say outside of the top three and Kyoto.

(And, ironically, while Hiroshima's fame stems from the fact that it was destroyed, Kyoto's popularity among tourists can be attributed to the fact that it wasn't ... so perhaps we should credit Secretary of War H. Stimson as the modern-day tourists' original influencer).