The Soviets were not a belligerent power and had treaty obligations to uphold to maintain their neutrality.

I've written about related matters here and here

I guess I wasn't as specific as I should have been. The island-hopping campaign was partially done so that we could get into range to bomb Tokyo, not with the atomic bomb but just with regular bombs. The B-29 was rushed into service so that we can have a bomber with the range to hit Tokyo. Curtis LeMay spent a year of the war coming up with a plan to try to bomb Japan from India. We put a lot of effort into figuring out how to drop bombs on Japan the whole War. My question is if we were allies with Russia why didn't we just use Russian airfields because they were so close to Japan?