3. The planners did not realize the extent of the Japanese kamikaze forces
The reason that the Japanese did not respond with air attacks against the bombarding ships and carrier strikes against their home islands is that they were converting the majority of their remaining air forces to kamikaze planes to be used against the invasion armada, which they knew from experience would be massive. The Japanese had nearly 10,000 planes available for kamikaze attacks (two thousand had been used at Okinawa, causing heavy losses) and several hundred suicide boats. Japanese planners intended to use the kamikazes primarily against troop transports in the invasion fleet in a sort of air-based banzai wave of attacks.
Although the number of kamikazes available was not known until after the surrender, the absence of air defenses launched against the Americans led Admirals Nimitz and King to argue against invasion and for blockade, aware of the heavy casualties which could be inflicted on both the Army and the Navy before any troops even got ashore in Japan. The Chiefs-of-Staff were unaware of the near readiness of the atomic bomb, but they were aware of the fire raids launched by LeMay from the Marianas. Further raids hit other cities from bases in China. George Marshall, urged by MacArthur, made the decision to move forward with planning for Operation Downfall.