
Larry C. answered 03/15/19
Computer Science and Mathematics professional
Likely for the same reason a proposal to drop the first on Mt. Fuji was also rejected - targeting something important to the population as a whole possibly could have stiffened their resolve instead to making capitulation more likely. The air defense of Tokyo was also probably much stiffer. Also, purely from a war standpoint, I suspect those two cities likely had more factories, drydocks and such vital to the Japanese war effort.