Direct annexation would not only really annoy and scare the West, it would make the nations revolt. Far better to semi-persuade the nations as much as possible.
Indeed, the Soviet Union had just come out of a war in which not only was its economy destroyed and large parts of its territory occupied for awhile, but in which it saw 23.4 million people killed, including 9 million soldiers, it's simply in no position to face the massive revolts that would ensue if it tried to annex Eastern Europe, hell I would'nt be surprised if it lead to the collapse of the USSR.
a horrible way for stalin to solve the "they'll revolt" issue would be to use russias vast land by making hundreds of new cities to deport everyone there, so that there's nowhere an ethnic majority or plurality able to claim it as their own, except russians.
but for that one stalin would need a hitler to stalin brain transplant.
That's pretty much impossible, you can't just move tens of millions of people like that.