France's coast seems to have been super well fortified and I'm curious if it was just too much of a logistical challenge to cover the distance through Russia. Or perhaps Japan just would not have allowed troops to pass through safely, fearing the US would attack Manchuria once inside the USSR.
This doesn't answer the whole of your question, but one factor is that Stalin wanted the Western Allies to open up a second front in Europe, as u/white_light-king/ answers here.