I could write one of my trademark horrifically long answers to this, but I'm a little pressed for time at the moment. Let me know if you need more details later. However, I've read more about the Korean/Russian perspective on this than the Japanese/American in the immediate aftermath of World War II, so I don't know as much as I'd like.

The TL:DR answer: The Soviet Union bordered the Korean peninsula, and the modern Russia still has it -- it's not very big (~12 miles) but it's there. Siberia also had a population of Soviet-Koreans who moved south with the Red Army and were extensively involved in the creation of the subsequent North Korean government. From the Soviet perspective, not only could North Korea serve as a useful buffer state between themselves and the American-occupied South, but the presence of a significant population of Koreans who were both committed communists and fluent in Russian meant that such a state could be easily controlled. Stalin was terrified of another invasion after World War II and sought to surround the U.S.S.R. with as much territory as possible, which accounts for a lot of what the Western allies saw as "land grabs" after the war.

Side note: Kim il-Sung wasn't a Soviet-Korean, strictly speaking, but he was advised by several (most of whom were later forced into exile, and some of whom may have ended up in the country's first prison camps, which probably began in the late 1950s/early 1960s). He wasn't the Soviets' first pick for leading a provisional government in NK after the war, but he appealed to them as someone with genuine credentials as a Korean anti-imperialist fighter. In essence, he was an easy "sell" to a Korean population that was humiliated by decades of Japanese imperial control. However, his Korean wasn't actually all that great by the time he returned to NK in 1945 (he'd been out of the country for around 20 years), and they wrote a bunch of speeches for him and even had to coach him on pronunciation. As you'd imagine, this isn't something that the official North Korean histories talk about.

Japan -- the better but less feasible option: By contrast, Japan lay across a sea from the U.S.S.R., and the Soviets didn't really have the naval resources to transport the masses of soldiers and supplies needed for an occupation. They capably fought Japanese imperial forces in Manchuria, but didn't have anywhere near the impact that the Americans could exercise on the home islands with both air and sea power in the Pacific. The bulk of Soviet military resources was still concentrated in the west. Japan and the U.S.S.R. (well, more specifically, Russia) also weren't on friendly terms after the Russo-Japanese War in 1904-1905, and it can be argued that the Soviets got what they really wanted out of that conflict via partnership with the Koreans anyway. All of North Korea's eastern ports on the Sea of Japan (most notably Chongjin and Hamhung) are warm-water, i.e., they don't freeze over in the winter, which was a huge strategic asset for the Soviets.

There's also the minor matter that after Japan finally surrendered, occupying Japan would have meant convincing the Americans to let them do it (probably a tough sell) or forcibly dislodging them (even tougher).

I don't know if that really answers your question, so let me know if I need to clarify. As an aside, this is one of the more compelling topics for people who write about and discuss alternate history. Had the Soviets split Japan in the same way that Korea and Europe were split, the 20th century might have been very different, and probably not for the better.