I believe the logic of the U.S. military planners was that they were worried the Japanese would assume the bomb was truly unique and novel. They didn't want the

I believe the logic of the U.S. military planners was that they were worried the Japanese would assume the bomb was truly unique and novel. They didn't want the Japanese to assume they could simply hunker down and expect a bomb every 2-3 months or so; the Japanese were already surviving equivalent damage from mere conventional bombing raids. That doesn't mean the U.S. couldn't have waited more (the Nagasaki bombing occurred while Hirohito's Supreme Council were discussing Hiroshima and the Soviet invasion), but I don't think they would have waited much longer either.