Though the Soviets have a superior ground force, it is the Allies Air Supremacy that wins out. By destroying all convoys and reinforcements that try to reach the Soviet front line, they starve them out and destroy them. By March, the Allies are in Siberia, having defeated the Soviet troops in Japan.
The Soviet Union is bleeding men and resources and urgently needs new ones. It has another army, but it is not clear if committing this army to Siberia can defeat the Allies. Instead, the Soviets could send them down into China, or launch a surprise attack into Japan. They need new resources, but should the problem in Siberia be addressed first?