I thought that it was because of Xenophobic bigotry displayed by George Bush and a number of other Republicans.
Perhaps BO made a lone stand in speaking truth to power against using the the bomb, but was ultimately overruled by John McNamara and Dick Cheney.
Or something like that. /sarc
In all seriousness, My Grandfather was overjoyed when they dropped the big one because it meant that he could head home and see my father for the first time, rather than head to Japan. Big difference in opinion between those who actually lived through the history, and those who are playing Monday Morning QB, with all the benefit of hindsight.
After my father passed away last year, I talked to a man who had been in Army basic training with him. They were in basic when Japan surrendered. They too were overjoyed, because they were training to go to Japan. They both ended up in occupied Germany instead. Without "the big one" neither I nor my siblings, our children or grandchildren might be here. While none of us might be especially famous or well-known, I like to think the world is a bit better off with us than without us.