Mainly because the Soviet Union did not attach importance to Finland at first, it can be said that the Soviet Union underestimated its enemies, which led to Finland's effective containment of the Soviet Union. Of course, it is also because Finland's terrain is very unfavorable to Soviet operations, and the Soviet army does not understand Finland's territorial terrain. It also hindered the Soviet attack on Finland. In addition, because of Germany at that time, the Soviet Union had no experience in dealing with Finland. This is also the reason why Finland took the opportunity to recover its territory.

Finland was only a small country at that time, with little military strength. However, the Soviet Union, with its huge military strength, ignored Finland. At this time, the Finnish army decisively chose the harassment policy and began to use the familiar terrain and weather to raid the Soviet Union with skis in the jungle. And it is a solo raid, they will suddenly appear in the Soviet camp, shoot for a while and then disappear into the snow. But this way made the Soviet Union unaccustomed.

At that time Finland regained its territory with the help of Germany. However, they did not take part in the war, but remained neutral in World War II. This also kept him from being attacked by other countries, because it was very dangerous to invade a neutral country at that time, because it would be strongly condemned by other countries all over the world. So the Soviet Union has no reason to attack Finland.

Of course, with the development of society, if you choose to attack a country at will, this kind of behavior will be strongly opposed by other countries and the international community. More importantly, the Soviet Union was later disintegrated due to economic development, and there was no ability to compete with force.