In keeping with Hersey's straightforward, journalistic style, the title gets right to the point—no frills, no metaphors, just the name of the place where the action went down. Of course, using "Hiroshima" as the title also draws attention to the scale of the tragedy and the fact that it affected an entire city, not just the six people that Hersey describes in his account.
Hersey frequently draws attention to the huge freaking scale on which the city was affected. For example, in providing statistics about the bombing's casualties, he writes:
In a city of two hundred and forty-five thousand, nearly a hundred thousand people had been killed or doomed at one blow; a hundred thousand more were hurt. (2.20)
The numbers don't lie—that's nearly the whole city. So, Hersey's title is a perfect encapsulation of the whole book, really, which uses simple, straightforward language to drive home incredible horror.