I am curious who these lots of historians are as the Soviet Union only redeployed about 3 divisions of Siberian troops, of which a grand total of one actually contributed to the Winter Battle at Moscow. On the other hand close to 300 divisions were organized and mobilized by the time the Germans got to Moscow, which one might suppose would play a much larger role in the German defeat.

As for why the Japanese chose not to attack the Soviet Union, please see a bit in my response here. For why they chose to attack Pearl Harbor, please see a response by /u/jschooltiger here.

Like much of the story of WW2, resources played a large part in the decision. Japan, as an island chain, largely lacked many of the natural resources required for both modern industry and especially modern warfare. There were two main schools of thought on how to obtain those. The Army favoured obtaining them from mainland East and Northeast Asia, while the Navy favoured seizing the European colonies in Southeast Asia and the South Pacific. It is worth noting at this point that the Imperial Japanese Army and Navy at this time enjoyed, or perhaps more accurately didn't enjoy, a rather intense rivalry, and worked together only reluctantly.

Owing largely to the fact that the Navy's plan required war with the European powers and that the Army was quite capable of getting things started on their own, the Army plan was the first to be implemented, using the Mukden Incident to seize Manchuria to create Manchukuo, and the Marco Polo Bridge Incident as an excuse to invade China. Neither action had required much input from either the Navy or the Civilian government, as the Army faction had managed a majority by that point. This continued for time as was, with the Japanese keeping the various Chinese factions on the backpedal and setting up four puppet states, but it didn't solve all their resource problems.

Interestingly enough, the Soviet Union, with its Siberian timber, gold, iron and coal, was considered as a possible avenue for growth under the Army Plan. So strongly, in fact, that the Kwangtung Army in Manchuria moved across the Mongolian border in 1939 to test the waters. The battles of Khalkhyn Gol proved that the waters were very violent indeed, and the Japanese Army withdrew, after which Japan and the Soviet Union signed a nonaggression pact, and the Army resumed its conquest of China while the Navy faction slowly gathered a majority in the Imperial cabinet back home.

Later that same year, Britain and France declared war on Germany, and the intensifying war in Europe, and especially the fall of France, presented the Navy with a golden opportunity. With everyone so busy in Europe, surely nobody would mind if they snatched up a couple orphaned colonies in the Pacific, right? As it turned out, someone did care very much: the United States. While the Japanese moved ships, planes and troops into Indochina in early 1941, the US continued to tighten its economic restrictions against Japan, banning Japanese shipping from the Panama Canal and, crucially, placing an embargo on oil and steel. Suddenly all the oil in the British and Dutch East Indies was no longer a particularly nice bonus, but a necessity. By the time Germany invaded the Soviet Union in mid-1941, the balance of power in the Japanese government had already shifted to the Navy and its interests, and the Army was not particularly interested in breaking the nonaggression pact anyway, after the thrashing received two years previously and there were not enough resources available to try out both plans.

This of course led to the invasion of SEA, beginning with the attack on Pearl Harbour, meant to pre-empt American involvement and following that, the conquering of Malaysia, Indonesia, the Philippines and finally Burma, to cut off the supply route into Nationalist China. Between these conquests, the now stalemated war in western China, the occupation of eastern China and the need to install a defensive perimeter in the central Pacific, attacking the Soviet Union was just simply no longer an option.

Another factor was that there already had been a series of undeclared border conflicts between the soviets and Japanese. These culminated in the Battle of Khalkhin Gol in 1939. The Japanese were beaten pretty badly, although the victory was a costly one for the USSR in terms of casualties.

The pre-war conflict helped convince the Japanese that they wanted no part of a fight with the Red Army. The Soviets certainly had bigger fish to fry in the face of a huge German invasion. So despite the fact that their respective allies spent a good deal of the war trying to get the USSR and Japan to fight, they didn't, until very late. The Soviet Union finally did declare war on Japan - on August 9 1945, three days after Hiroshima.