There were multiple reasons why the United States did not attack Japan from the north. The most important, of course, was that on 13 April 1941, Japan and Soviet Union signed a non-aggression pact. At the time, the Pact was very much in the interests of both powers, as it freed up Soviet troops from guarding the Far Eastern frontier to deal with the German invasion, while it also secured Japan's northern flank, allowing Japan to move south against the European colonies. The Soviet-Japanese Neutrality Pact included provisions that each party would respect the territorial integrity of the other, and that each would remain neutral in the event of conflict between one contracting party and any other power. If the Soviet Union had allowed American airbases to be established in its territory, from which the U.S. sought to attack Japan, it would be a clear violation of the Neutrality Pact. Even though the Germans might no longer be at the gates of Moscow, they were still a massive threat, and the Soviets were unwilling to open a new front in the Far East until Germany was defeated. While it would seem strange that the Soviets would be constrained by a piece of paper, it's important to note that Stalin appeared to be very concerned with ensuring that the Soviet Union be seen as operating within bounds of international law. Even in 1945 when the Soviet Union attacked Japan in breach of the Neutrality Pact, Stalin eagerly sought to frame the breach as one caused by request of the United States and Britain. The illegality of such operations would form a major barrier to any proposed American bombing campaign from Soviet territory.

Beyond the illegality of such an operation, there were significant technical limitations on operations from the Russian Far East against Japan. For one, the only real port in the Russian Far East in the 1940s was Vladivostok, which was nearly completely isolated from the Pacific by the Japanese archipelago. Even in 1945, when the majority of the Japanese Navy lay at the bottom of the ocean and Allied submarines operated in the Sea of Japan with impunity, sailing massive cargo convoys to Vladivostok would involve passing through straits controlled by Japan, which would be extremely vulnerable to attack. Beyond this, the Northern Pacific was an extremely hostile operational environment. It simply wasn't possible to move the supplies needed for large scale operations through the often storm wracked waters of the Northern Pacific, as both the Japanese and Americans found via their operations in the Aleutians. Supporting operations in the Russian Far East via shipping to European ports was also non-viable. Not only would the supplies have to travel the great distance across the Trans-Siberian Railway, but the railway itself was extremely vulnerable to attack from Japanese forces in Manchuria.

Finally, there were political considerations. While the early history of American-Soviet conflict that would grow into the Cold War are not my specialty, it was quite apparent well before 1945 that there was potential for conflict between the Soviets and the United States following the defeat of the Axis powers. To give the Soviets access to a substantial number of American forces, isolated in the Soviet Far East, dependent on Soviet cooperation for supplies and everything else would simply be politically unacceptable.

I hope this has helped to answer your question. Please feel free to ask any follow ups.