The best way to understand the motivations of the people who chose the targets is to read their own reading, from the time. Here are the meeting notes from the Los Alamos Target Committee, May 1945, when they came up with the first official list of targets. (As you will see, it does not match up with the final target list — Kyoto and Yokohama were removed from the list, Nagasaki was added.) The section titled "Status of the Targets," "Psychological Factors in Target Selection," and "Use Against 'Military' Objectives" are the most evocative in my opinion.