Allied conferences during WW2 at Yalta and Potsdam ensured that the USSR received support from the USA and UK to occupy these eastern European countries and play a role in their reconstruction and post war transition.

There was disagreement and miscalculations amongst western leaders over how influential the USSR would end up being in this post war transition. They may have expected a agree of cooperation between eastern Europe and the USSR, but not the outright puppet states that resulted.

The US and UK, had already accepted the USSR's annexation of the Baltic states and parts of Poland, but Stalin rightly anticipated that the outright annexation of other nations that has not historically been a part of the Russian empire would have sparked another war that the USSR was not in a position to fight. He ended up receiving most of what he wanted through these puppet states rather than direct Soviet rule.

sk9592 tells part of the story, but he is making a common, erroneous assumption about what constituted a "Soviet satellite" and has thus, like many people, looked only to Soviet and Allied history for an answer regarding Polish, Hungarian, Czechoslovak, etc., histories.

First and foremost, a Soviet "puppet" state was not, as you might assume, a nation that did Moscow's bidding and was thus independent from the Soviet Union only as a technicality. These nations had their own domestic policies, and these policies sometimes clashed with those promoted by the Soviet Union. In Czechoslovakia in the 1960s, for example, before the Prague Spring, the Communist Party began allowing enterprises to keep a portion of their profits for themselves, allowing the workers to receive bonuses and the managers to receive even larger ones. They struggled to implement this plan, but not because of Soviet interference. Czechoslovakia was its own nation.

OK, with that being said, we should look at what made these nations "puppets": obviously no nation would survive as a "socialist" nation without the massive influence of the Soviet Union, and thus the leaders of those nations, almost all of whom were themselves Marxists-Leninists to some extent and who believed their actions justified given the threat of Western imperialism (i.e., WWII), would lose power without the support of the Soviet Union. Furthermore, the leaders of those nations knew, from experience, that if they strayed too far from the Soviet line, the Soviet Union would use its power to make a regime change in that nation. This did not mean, however, that these leaders, their governments, or their nations would ever allow for total Soviet domination. These "puppets" could not be absorbed into the Soviet Union by peaceful means. If the Soviet Union wanted Hungary, Poland, etc., they would have to invade and occupy--a daunting task even before considering the West's response (which, as sk9592 pointed out, could mean war). Furthermore, occupation of these nations would have reduce their benefit as buffer states for the Soviet Union. The Soviet Union, especially Stalin, wanted allied buffer states to separate the Soviet Union from Germany and the West. Had the Soviet Union occupied these nations instead of coercing them into an alliance, then they would have lost the benefit of having this buffer zone.

In short, Soviet satellite states were not true "puppets" and would not allow themselves to be integrated into the Soviet Union without a fight, and the Soviet Union would have, in any case, lost some of its perceived strategic advantage had it occupied and integrated those nations--assuming, of course, that the occupation of these countries would not have led to a general war with the West. Even shorter: The Soviet Union could not, and had little reason to, annex these nations.