In part because the first time they tried they got a bloody nose for no gain.

In the Spring of 1939 tensions and border skirmished boiled over in the Khalkhin Gol campaign. Roughly Corps sized units of the IJA and Soviet Army duked it out until late August. The Soviets came out as the clear winner, early Japanese advances were thrown back and the counterattack, lead by the soon to be famous Zhukov, surrounded and destroyed a Japanese infantry division.

The Soviets enjoyed larger superiority in important factors such as motor transport, aircraft, and if not qualitatively than certainly had numerical superiority in AFV's.

And soon after a ceasefire was signed the Molotov-Ribbentrop Pact was announced, making them sort of half allies. This defeat, at a time when Japan's ambitions were still very much focused on the promise of China, and the far more ripe, Dutch East Indies and Indonesia. This areas offered many more resources for the taking than remote and underdeveloped Siberia. While important intelligence leaks also allowed Stalin to be sure that even in the dark days of late 1941 Japan was more interested in the easier targets vs trying to attack the Soviet Far East again, the situation in Europe not withstanding.

Thus in 1941 an official non aggression pact was signed with that would hold until August of 1945 when the Soviets stormed over the gutted and withered IJA presence in Manchuria and seize a huge swath of the old Tsarist empire down even tot he Korean Peninsula.