As this is an atomic bombing question, we shall of course refer to u/restricteddata on this matter. More can always be said from others on this matter, so if anyone else would like to address how the target list was developed, please don't let this post stop you!

For the meantime, OP, here's some of u/restricteddata's previous posts on target selection:

• On the list of targets as of the second meeting of the Target Committee;
• and on Kokura, which was the intended second target.