The war was launched right in the middle of The Great Purge which meant that many officers in the Red Army had been killed. That means that many of the officers in the Red Army were inexperienced and/or not respected. Like in many realms of the Soviet Union, loyalty was valued more than competence. To put it simply, one of the biggest reasons is because the Red Army leadership was not good.

Russian soldiers have olive camouflage at the beginning as did the tanks, so that meant they were pretty much beautiful green invaders in a white wasteland (joke). It was also in the negatives constantly, reaching as low as -45 F at one point. While both were equipped pretty well, in terms of winter gear, the Finnish had heated building to retreat to, and use as bases/ both medical and military. The Soviets on the other hand had to take destroyed buildings or set up bases and these were not warm by any standard. It's ironic that the Russians have always used winter against their enemies, only to have it used against them by the Finns.

Finnish morale and tactics were also better during the war. Finland was very defendable and didn't suffer from purges which made their military leadership inept, they were also very keen on defending their homeland. They knew the Soviets treated their people badly, and did not want that to happen to them. The reason that Finns were better in winter warfare is simply better preparation and equipment. The Soviet Union is enormous and therefore they must fight in Steppes, mountains, hills, marshes, however Finns are predominantly marshes, and cold.

So to sum it up, very simply, bad weather, bad leadership, bad tactics, bad materiel's led to Soviets BARELY beating the Finns. Had a purge not occurred, and better preparations for the offensive been made, it would have ended much quicker.