Here is an answer to a similar question I answered previously. The short answer is that Stalin was focussed on defeating Germany, not Finland. The Soviets controlled relatively little Finnish territory at the end of the Continuation War, and so had little means to control the country through force of arms. Finally, blatant conquest of Finland would have been seen as antagonistic by the western Allies, especially as peace terms with Finland had been set at the Tehran Conference in 1943.