The first Bombing occurred on Aug 6th 1945 in Hiroshima and the second occurred three days later. Many Japanese died. Then on Aug 15, Soviet Union declared war on Japan. After the Soviet Union declared war, Japan surrendered. I believe the largest munitions plants and an area that had a large number of military units were in Hiroshima making it the best target to cripple the Japanese. Japan didn't surrender at the time of the first bombing.
I think your best answer would be that : The Soviets declared war on Japan because that is when Japan officially surrendered
