The various conferences before the war were part of the reasoning for this. The Yalta conference in particular was one the guaranteed the independence of a few of these countries and in other cases also called for elections inside them. While generally the agreement thats not truly what happened as during WW2 proper the Soviets on multiple occasions made it known that they were not up for annexing all of these states. They most notably allowed the Polish uprising in Warsaw to fail. This is essentially a strategy of letting two of your enemies grind each other down. This allowed the Soviets to not only fight the severely weakened Germans, but also put in whoever they wanted in Poland at the very end of the war.

Generally you could also just consider how the USSR was at this point. By the end of World War II the Soviet Union was a scared, but triumphant nation that was not willing to go through this a second time. So rather than annex all this new territory and cause your own problems by including a bunch of random and large ethnicities that you cant just "deport" to Siberia you create a belt of allied/satellite states which act as the buffer in this case to prevent what happened in WW2 to happen again. This eventually evolved into the Warsaw Pact (which in itself was an answer to NATO) and made it somewhat easier for the Soviets to manage. The Soviets were invested into these countries, but they would hence do alot of their own development which considering the USSR was in recovery mode until the late 60's early 70's (this is debated still afaik) this was preferable to annexing a bunch of new territory with non related ethnicities. For the most part the explicit goals of the USSR only ever extended towards regaining its territory within the former Russian empire. This was made most obvious by Polands historically eastern areas correlating with the old Duchy of Lithuania which was part of the Commonwealth back in the day, being removed and grafted onto Russia since the majority of the populations here were Belarussian and Ukrainian/Ruthenian. And with the USSR already controlling most of Ukraine it was a no brainer to secure more of the East Slavic peoples.

Finally ill add that the USSR was just simply not in a position to violate all the yalta agreements even if it wanted to. The United States was the only country with nuclear weapons, and the USSR was already suffering from manpower issues due to its fight with Germany. That is just a recipe for disaster for the USSR and was content (somewhat begrudgingly i might add) with a series of buffers or satellites to prevent a Future Barbarossa essentially. Combined with the resources it would have taken to forcibly integrate all this stuff it should be noted that the Warsaw Pact had a member withdraw and the Soviet union crushed the revolt. This was in Hungary in 1956 and another invasion occurred in Czecoslovakia later as well.

I can answer more if youd like i was just giving this a rough over since i just woke up and saw this and wanted to write about it :P

Sources: Courtois' Black Book of Communism

UN General Assembly Special Committee on the Problem of Hungary (1957) Chapter II. A (Meetings and demonstrations), para 54 (p. 19)

"Document #81: Transcript of Leonid Brezhnev's Telephone Conversation with Alexander Dubček, August 13, 1968". The Prague Spring '68. The Prague Spring Foundation. 1998. Retrieved 23 January 2008.