Japan was so shocked of the Red Armies over whelming successes in the Manchuria that they believed that the USSR could successfully invade the Japanese mainland.
Japan never believed that the Americans or the British could pull of a successful invasion of Japan.
the biggest factor in the Japanese capitulation to the US is the fear from the Soviets, Hiroshima and Nagasaki was barely mentioned in the War Cabinet´s log, which suggests that wasnt the main reason for surrender.
The main reason we focus on the:
is because this was the first time they were ever deployed in war. While nothing special at that time since cities could be destroyed without them anyway (fire bombing, V2 rockets, ect.) The Cold War built the nuclear fear inside people and the myth that their usage instantly capitulates countries greatly contributed.