If you read about what was going on within the Japanese leadership, basically their only remaining plan was to leverage what they thought was the goodwill they had with the Soviets (for not invading when the USSR was in dire straights) to get Stalin to mediate a peace agreement with terms something less than unconditional surrender. There is evidence that Stalin strung them along, hoping to keep the war going until his forces would be able to invade Manchuria and gain important territory for the Soviet Union.
With the spectacularly successful Soviet invasion the game was totally up. All percieved diplomatic leverage was gone, and the last real standing force of the Japanese military had been soundly defeated. The atomic bombs certainly hastened the surrender, but Japanese cities had been destroyed before. The real game changer was the Soviets avenue for a negiotiated surrender had been cut off.