Influence of a Japanese attack in Siberia

For the same reasons the United States didn't supply China with a great quantity of tanks. IIRC, China received 35 M-4 Shermans and 44-48 M-3/M-5 Stuarts.

China was simply incapable of maintaining a large armored force in the field for any amount of time. Her logistical network was in deplorable condition, which meant that large armored forces could not be transported by rail or road. Nor was China's industry capable of maintaining a continuous supply of spare parts to keep any tanks in a state of good repair. Likely, China's fuel situation was also inadequate to keep any mass of tanks in a steady supply of fuel.

Any sizable Chinese armored force would be broken-down and out of gas right quick.

Actually, the Soviets would have...and did, supplying Nationalist China with almost 90 T-26 tanks, and many armored cars. Not to mention a large number of aircraft and pilots.

By keeping the Nationalists in the fight, only served to preoccupy and weaken Japan. Also, Nationalist purchase of Soviet war material served to keep much needed cash flowing into the Soviet Union. Further, it allowed Soviet military personnel to gain combat experience, as well as test their leadership abilities.

Politics goes right out the window whenever money or needed goods is involved.