By the time of the Hiroshima bombing, the U.S. was out of military targets and running out of civilian targets that hadn't already been thoroughly pulverized by

By the time of the Hiroshima bombing, the U.S. was out of military targets and running out of civilian targets that hadn't already been thoroughly pulverized by saturation bombing. The impact of the first atomic bomb drops were therefore largely psychological. Consider the Japanese perspective. Pretty much every military target and major urban center is already in ruins, and suddenly the enemy starts dropping an entirely new kind of bomb? The Japanese weren't scared of the handful of nuclear bombs the U.S. had managed to build. They were scared of the idea that the U.S. would start saturation nuking the entire country, even if the U.S.'s nuclear stockpile was purely symbolic at that stage. As for bat bombs, given that the conventional bombing of Japan was so successful that nuclear weapons were a purely psychological threat, it's not surprising that "bat bombs" were deemed superfluous. Interestingly enough, the "bat bomb" idea might be due for renewed consideration. Nuclear weapons are now considered last resort. Cluster bombs are banned by many nations and considered rather despicable to use due to the fact that the duds remain a threat to civilians long after the conflict is over (Note: The U.S. has not signed the ban and still uses cluster bombs). A "bat bomb" full of mini-drones might provide superior coverage and accuracy to cluster bombs. One such bomb could cover a larger area and target specific structures. Drones could fly their payloads directly inside buildings to structural weak-points, greatly multiplying the impact of their munitions. There would still be duds of course, but, by reducing the amount of ordinance needed to hit all targets in a given area, the impact of unexploded duds on civilians would hopefully be reduced.