The proximal answer is that Japan and the Soviet Union had concluded a neutrality pact on 13 April 1941. That agreement followed on the heels of a series of border clashes during the 1930s between Soviet forces in the Far East and Japanese forces in Manchuria (then the Japanese puppet state of Manchukuo). These clashes had culminated in the Battle of Khalkhin Gol (aka the Nomonhan Incident) at the border between Mongolia (then a Soviet-backed Communist state) and Manchukuo. Japan launched an assault on the Soviet-Mongolian positions in July 1939, but by August 1939, the Soviets had gained the upper hand and launched a counterattack that forced Japan to accept a cease fire, followed by the neutrality pact in 1941. Of course, Japan could have unilaterally abrogated that pact in the same way that Germany ultimately did with the Molotov-Ribbentrop Pact, but ultimately decided not to, focusing instead on the so-called "southern path". They weren't obligated to attack the USSR under the Tripartite Pact, even though it obviously would've helped Germany.

Most analysts attribute the Japanese decision not to attack the Soviet Union in 1941 to the psychological impact of the defeat at Khalkhin Gol on the Japanese government and military. Japan had harbored imperialist ambitions toward the Russian Far East, enticed by the natural resources of eastern Siberia, but after Khalkhin Gol, their attention shifted southward, toward Southeast Asia and the Pacific islands. Japan did consider launching an attack against the Soviet Union in concert with the German invasion in the west, but they ultimately decided against it, a decision often attributed to the experience at Khalkhin Gol. It's also worth remembering that at that time, Japan was embroiled in a grueling war in China that had already dragged on for a decade and showed no sign of ending, and that Japan's relations with the United States were also deteriorating, increasing the likelihood of a war between Japan and the US; the Japanese were well aware that they didn't have the manpower or industrial capacity to fight simultaneous wars against China, the US, and the Soviet Union, so maintaining Soviet neutrality was beneficial to them for the time being.

Obviously the Japanese decision not to attack the Soviet Union had major implications for the war between Germany and the Soviet Union. Thanks to their network of spies in Japan (most notably Richard Sorge, who had given Stalin the exact date of the German attack) as well as SIGINT, the Soviet government had been informed in September 1941 that Japan wasn't going to attack the Soviet Union in 1941 (and would not attack until Moscow had fallen and the Kwantung Army had a decisive numerical advantage over the Soviet forces in the Far East). This intelligence allowed the Soviets to transfer forces that had been tied down protecting the border with Japan to the western front, which ultimately proved decisive in the Soviet victory at Moscow at the end of 1941. By that point, Japan was at war with the US and had bigger problems to deal with than fighting the Soviet Union; there is some speculation that Japan would have considered invading the Soviet Union in the event of a German victory at Stalingrad (Sorge had reported in 1941 that Japan would attack the Soviet Union if the Germans secured a port on the Volga), but obviously that didn't happen and no invasion occurred.

On the Soviet side, there wasn't really any intention of fighting the Japanese until the war against Germany had concluded. At the Yalta Conference in February 1945, Stalin promised Roosevelt that the Soviet Union would invade Japan within 90 days of the end of hostilities against Germany, which would give it time to resupply its forces and transfer them from Europe to the Far East. True to his word, on 9 August, three months after VE Day and three days after the atomic bombing of Hiroshima, the Soviet Union invaded Manchuria, making rapid progress and occupying most of Manchuria and northern Korea before Japan surrendered on 15 August. The relative importance of the Soviet invasion vs. the atomic bombings in the Japanese surrender is still debated, but it no doubt played a role in forcing an end to the war in the Pacific theater.

Sources:

Text of the Soviet-Japanese Neutrality Pact of 13 April 1941

Anthony Beevor, The Second World War (Weidenfeld & Nicolson, 2012)

Alvin D. Coox, Nomonhan: Japan Against Russia, 1939 (Stanford UP, 1990)

David Glantz, The Soviet Strategic Offensive in Manchuria, 1945: August Storm (Frank Cass, 2003)

Robert Whymant, Stalin's Spy: Richard Sorge and the Tokyo Espionage Ring (Palgrave MacMillan, 2006)