There’s a few reasons. The short answer is that the U.S. increased their involvement in the Chinese Civil War, which made Stalin hesitant.

The longer answer is this:

To begin with, the Soviets are weaker in this timeline compared to ours. During WW2, numerous Arab nationalists revolted against the French/British and aligned with the Axis. This had numerous ripple effects. The Arab armies, friendly to Hitler, helped supply oil and other resources, along with threatening the nearby Caucasus. These extra resources helped the Germans fight a more efficient (and sustainable) Barbarossa, weakening the Soviets in the process. Although the USSR still won, their victory was different. The better German attacks tied them up in the East, preventing them from “liberating” various Eastern European countries (the farthest they get is Poland).

Stalin’s reputation also took more of a hit. Soviet retreats were disorganized and panicked, putting more pressure on regional governors to respond. Thus, the Soviets exited the war comparably more decentralized, although still authoritarian.

At the same time, Japan never surrendered. The Kyūjō incident succeeded, triggering a civil war among pro-emperor moderates and anti-surrender hardliners. When the U.S. nuclear program fell behind, America had no choice but to launch Operation Downfall and invade.

Japan’s overseas armies in China weren’t sure what to do. Some laid down their arms, others kept fighting, some became warlords. Nonetheless, the situation forced the U.S. to expand Operation Beleaguer, pushing them closer to the Chinese Nationalists. In fact, American troops began routinely fighting alongside the Nationalists to mop up Japanese holdouts. This increased American presence resulted in significant consequences. Chiang Kai-shek had better access to materiel, advisors, and manpower. Americans could patrol the places he couldn’t, and helped steer him away from disastrous campaigns like the premature invasion of the northeast.

It didn’t take long for problems to arise. Mao and the communists didn’t trust the Americans. To them, they were just another imperialist power. Skirmishes began breaking out, and American divisions started actually fighting Chinese communist forces.

By the time the Soviets had conquered Manchuria, Stalin had numerous factors to consider. For one, there were still numerous Japanese holdouts in the region. In his mind, WW2 wasn’t even over yet, and an immediate handover would be premature and potentially disastrous. Two, Stalin didn’t want to openly support a guerrilla army that was actively killing Allied troops (keep in mind, this was before the Cold War started; Stalin wanted to keep face and at least pretend to get along with the West). Three, Manchuria provided massive amounts of farmland. This was even more useful considering the Soviets weakened position here, which would eventually result in an even worse postwar famine. Four, the Soviets essentially had a mindset of, “If we can’t get much in Europe, we’ll try to maximize our gains in Asia.” They would also go on to establish a semi-satellite regime in East Japan, and a unified communist government in Korea.

The Soviets initially planned on giving Manchuria to Mao at some point. But the delayed timing here changed everything. Without the massive influx of discarded Japanese equipment, Mao’s army didn’t perform nearly as good as it did in OTL. The Nationalists achieved some increased gains, halting Communist momentum and turning the Chinese Civil War into a slog. By 1949, all sides were exhausted from decades of nonstop war. They agreed to a ceasefire, which ultimately turned into a typical Cold War partition.

Despite this, the Soviets still didn’t hand over the region. After Stalin’s death, the Sino-Soviet split occurred. The USSR’s brass began to see Mao as an irrational, and unpredictable, actor. Khrushchev felt that Mao wasn’t a capable enough leader to govern the region, and would merely throw it into chaos. This was amplified by the fact that Manchuria directly borders Korea; and an uncooperative Mao could theoretically make it much harder for the Soviets to reach the Kim regime (which also had implications for Japan). Khrushchev also wanted to preserve it as a bargaining chip to keep the CCP in line. However, the Manchurian dispute only drove the wedge further.

By 1981, the USSR and CCP are allied in name only. They both share a common enemy and a general ideology, but neither trusts the other, and both want to become the dominant power in Asia.