What were the main reasons for the peace treaty (admittedly with good terms for the Soviet Union) rather than invasion and occupation of the country.
Following the Moscow Armistice, two high-ranking officers in the Operational department of Finnish Military HQ, Colonel Valo Nihtilä and Lieutenant Colonel Usko Haahti, started planning countermeasures against a possible Soviet occupation of the country. They came up with the idea of decentralized storage of light infantry weapons, so that in case of occupation, an immediate guerrilla war could be launched.
During the demobilization, an organization responsible for hiding the equipment was created and war materiel and other supplies were given to them for safekeeping. A total of 5,000–10,000 people participated in the operation. It was planned that they would cache supplies for 8,000 men, but the participants worked so eagerly that it is supposed they hid enough for 35,000 soldiers.
In 1945, Interior Minister Yrjö Leino exposed a secret stay-behind army which was closed down (so called Weapons Cache Case). This operation was organized by Finnish general staff officers (without foreign help) in 1944 to hide weapons in order to sustain a large-scale guerrilla warfare in the event the Soviet Union tried to occupy Finland in the aftermath of the Continuation War.
Operation Stella Polaris was the cover name for activity in which Finnish signals intelligence records, equipment and personnel were transported into Sweden after the ending of the Continuation war in 1944. The threat of Soviet occupation was considered too likely and an operation was formed to support guerrilla warfare in Finland after occupation.
TL;DR: Supposedly, Stalin knew that the Finns had a system put in place to continue a guerrilla war, and he was already fed up with the previous results of the Finnish front.
Firstly, bear in mind that the Soviet-Finnish peace treaty was signed in 1944, when the Soviets were still at war with Germany (albeit winning). Finland still fielded significant military forces and, while there was no question that the USSR would eventually win in any prolonged war, it would have required a lot of resources to do so - at a time when Soviet military resources were needed for the push into central Europe. The Soviets also believed, quite probably correctly, that were they to try invading Finland proper, Finnish resistance would stiffen due to civilian mobilisation and pre-prepared defenses.
So the Soviets faced a fairly stark choice in 1944. Sign a favourable peace treaty that left Finland independent but neutralised it as a threat to the USSR for the forseeable future, granted the Soviets significant concessions (reparations, basing rights, territorial transfers) and freed up a large number of Soviet soldiers for operations in Europe. Or alternatively, press for annexation, which would require transferring resources to the Finnish front when they were badly needed elsewhere. It's hardly surprising the Soviets took the first option.