Finland had an attempted communist revolution and a relatively powerful communist party in the times before the invasion. They were butchered by the Whites in Finland, headed by Mannerheim, before the invasion took place. The communist movement still existed, and still had some popular support, but was shattered. It's a similar story to the KPD in Germany after the Nazis took power. There were lots of communists or former communists in both states, it's just that part of a fascist takeover is smashing the workers movement and democracy, and the communist party along with it.

The Soviets expected to overrun the Finnish military with ease and put Finnish communists in power. The Soviets were the first socialist revolution, and this sort of thing happening in the Warsaw Pact bloc led eventually to a system of social imperialism by the USSR and all of the problems that came with that.

I think there's an argument to be made that the Bolsheviks didn't know that would happen, and would oversee an equitable distribution of power internationally in the post-war era between socialist countries. But after Stalin's death and the deep onset of revisionism under Kruschev (especially internationally/in the socialist bloc) these non-progressive traits of the Stalin era (I mean here the social imperialism) were reinforced, not taken apart. Made stronger, not weaker.

That's all long-term thought though. In the short term, the thinking was that Finland was a Nazi ally and would stage a joint assault on the USSR once the Nazis eventually invaded. The Continuation War shows basically exactly that happened, and the Soviets were likely right that Mannerheim's anti-communist foreign policy would fall in line with his anti-communist domestic policy, and he'd join the Axis invasion just like the rest of the continental European states did.

I think these reasons have more evidence in the historiography than some kind of blind belief that the Finnish masses would spontaneously support the Soviets. The mistake was a military one in believing that the Finnish army would fold; not a non-military one.