The U.S. sure did a lot of time-consuming island hopping to get to Japan's mainland, but Russia is right next to Japan on the west. Setting up a front next to Japan in Russia would have been very difficult to impossible early in the war, and Russia absolutely did not want an eastern front while fighting Germany, but what about in 1943, 44, and certainly 45? Russia had a nonaggression pact with Japan, but they could have broken it anytime they wanted (as they did in 1945). Did the U.S. or other allies even try to get Russia to let them set up across from Japan?
There are several reasons
Up until late June/July of 1944 Germans still occupied large parts of Soviet territory like Belarus, parts of Ukraine and Baltic republics. Only after Bagration (started on June 22/23) and Lvov-Sandomierz (started in July, ended in August) we could say that main part of USSR was liberated, although some German pockets like Courland remained until the end of war.
In addition to this, Soviets really faced manpower shortages late in the war. So much that they had to conscript 17 year old boys, but also various "elements" from recently liberated territories. There were many Ukrainians, Belarus, Lithuanians, Latvians, Estonians… not really willingly fighting in the Red Army, some of them actually working for Germans previously. But they simply switched sides and have gone with the flow, better to be a Red Army solider than face NKVD and SMERSH back home. Of course, Soviet officers understood that very well, but they had no other choices, so they were also going with the flow as long as they were advancing and German defeat was nearing. Soviets also armed Poles (who traditionally hated Russians and Moscow) and Bulgarians and Romanians (recently switched sides) just to keep war momentum going forward. Finland also got away relatively easily because Soviets did not want to waste time and resources. All of this is important, because simply saying, until Germany was defeated, Soviets could not afford war with Japan. In fact, USSR promised on Yalta to attack Japan 3 months after the end of war in Europe.
Another thing to consider was could US and UK afford those 50 divisions ? Obviously, not without either stripping their forces in Europe (not in USSR interest) or from other places in Pacific which would slow down offensives there. Of course, stationing US divisions on Soviet soil would breach neutrality. Japan would have no other choice than to try to prevent this, both on sea and on land. Kwantung Army was gradually weakened during the war, now the process would be reversed. Like it or not, they would have to attack Soviet forces in Far East which were also gradually weakened during the war. Logistics would also play its part. Soviets didn't have enough food even for themselves, Japanese would do their best to block Vladivostok etc… Overall, buildup would be slow and comparatively ineffective compared to actual historical campaign of island hopping.
Ideological differences would also come to effect. Soviets generally mistrusted Western Allies and same could be said in vice versa (especially British). Soviets reluctantly agreed to shuttle bombing, but 50 divisions would be too much, especially in the Far East. There was no way to ensure these divisions would leave in the end of the war, and Far East intervention from Civil War could repeat itself. Of course, exposing population to capitalism of US/UK forces would also be detrimental to Soviet power - workers and peasants could learn about much better living conditions in capitalist countries.
Finally, when Germany collapsed, as mentioned before, Soviet Union did not need those 50 divisions. As promised, they did attack Japan exactly 3 months after end of war in Europe. Soviet invasion of Manchuria crushed Japanese forces with relative ease. Atomic bombings did force Japan to surrender, but in alternate scenario without them, this new front in Manchuria and Korea would simply be too much for Japanese. US and UK of course knew that, and it could be even said that Soviet attack was some kind of backup plan in case that atomic bombs do not achieve desired result.