At Truman's urging and insistence. There was no need. Consider also the 100 American carriers in the region at the time and what Russia would have had available to even try to get to Japan had we not wanted them there (inner tubes, canoes, rafts...)...
Russia had scheduled the invasion of Hokkaido and were in the process of moving people when the bombs were dropped.
We could have ended up with North and South Japan. It was very close.
We have never had 100 carriers.
Where di you get this number?
Most of the aircraft that were scheduled to take control of the skies were to come from Okinawa gound bases.