I've answered a very similar question here. The short answer is that by the time Germany launched its attack on the Soviet Union, Japan had already committed itself to moving south, securing a non-aggression pact with the Soviets in order to secure Japan's northern flank. The German invasion indeed was advantageous for Japan as it removed any potential for a Soviet attack on Manchuria while they were in a death struggle against the Germans. While the Japanese considered denouncing the non-agression pact to join in (including Foreign Minister Matsuoka who has negotiated the same non-agression pact he now wanted to denounce), the defeat suffered and Nomonhan seemed to indicate that any campaign against the Soviets would not be a short one, and the Japanese ultimately opted for only moving against the Soviets if the Soviet Union was already collapsing.