The Soviet Union saw Germany as a potential and dangerous enemy. Leading up to WW2, the Soviets had tried to secure a firm military alliance with Britain and France against Germany. This didn't happen, and the Soviets opted for the Molotov-Ribbentrop Pact instead. This didn't change the view of Germany as a potential and dangerous enemy, but from the Soviet view, the war between Germany, France, and Britain that followed Germany's invasion of Poland removed the immediacy of that danger. The Soviet Union had time to prepare for the future.

Part of this preparation was the Soviet invasion of eastern Poland. Another part was the takeover of the Baltic states (Estonia, Latvia, Lithuania). Yet another part was the diplomatic pressure against Finland, followed by the Winter War. Two key Soviet demands on Finland were moving the Finnish-Soviet border on the Karelian Isthmus further from Leningrad, and the establishment of a Soviet military base on the Hanko Peninsula in SW Finland. These were clearly military demands, related to the defence of Leningrad and control of the Baltic. Both would contribute to a better position in a future war with Germany.

From mid-September to early October, the Soviet Union was busy invading Poland (and before that had been fighting Japan at Khalkhin Gol, with heavy fighting in July and August). There was no time to feasibly attack Finland before November. Could the Soviet Union wait? How long would WW2 last? After Poland was devoured, how long would France and Britain remain at war with Germany? Stalin hoped that the war would continue, but this was not certain. The Soviet position would also be hurt by Scandinavian entry into the war.

Added to this urgency was a belief that an invasion of Finland would be a walk-over. If a three month rather than three week war had been expected, it's quite possible that Soviet invasion plans would have been postponed (despite the risk of a changing international situation making an invasion too dangerous).