The Japanese were terrified of the Soviets following the Battle of Khalkhin Gol. The Soviets were able to throw massive amounts of men and equipment into the battle and, despite some minor things like a lack of radios in Soviet aircraft, the Soviets showed themselves to be superior in pretty much every way. Their armored cars - not even their tanks - outclassed the Japanese tanks. Their artillery was more numerous and modern than the Japanese, as well as significantly better trained. The Soviet army was more motorized than the Japanese Army and their logistics were overall better - during the campaign, despite operating from a railhead a four-day drive away (as opposed to the half-day drive for the IJA), the Soviets were able to build up forces and set up an artillery barrage that not only outshot the Japanese artillery, but even ramped up in intensity while the Japanese guns ran short on ammunition. The biggest shortcoming was in aircraft - the IJAAF was superior in terms of individual quality of aircraft and pilots, as well as doctrine - but the Soviets more than made up for it by being able to field significantly more aircraft to the point that the IJAAF just couldn't compete.

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At Khalkhin Gol, the Soviets nearly annihilated the Japanese forces present and were poised to sweep through Manchuria nearly unopposed had they wished. The engagement caused Japan to realize how outclassed they were and left them terrified of a Soviet invasion. They'd ultimately work towards a non-aggression pact with the Soviets, and they never really considered war with the Soviets ever again. Obviously, the situation in 1941 may have been significantly different - it's unlikely the Soviets would have been able to field a force like they did at Khalkhin Gol with the Germans at the gates of Moscow - but by that point Japanese priorities had already shifted. Japan had invaded French Indochina and was even more embroiled in China than they were in 1939, and Japan's focus was now firmly in the Pacific.