As I understand it the USSR was set up to be ever-expanding, with new communist states joining the Union as Soviet Socialist Republics, to eventually cover the globe (and then wither away etc. etc.). In the Cold War was this idea officially abandoned or pragmatically ignored, as none of the Eastern European satellites were absorbed as members (except the 3 Baltic states)?
EDIT: Relevant part of preamble to the 1924 USSR Constitution: "...that access to the Union is open to all Republics already existing as well as those that may be born in the future... and mark a new decisive step towards the union of workers of all countries in one world-wide Socialist Soviet Republic." [http://www.answers.com/topic/1924-constitution-of-the-ussr]
I don't have a particularly comprehensive answer, but at least one factor would have been the rise of the United Nations.
In a format where most nations have the same voting power, Stalin was worried that the USSR would be outvoted by the wide sphere of influence that the UK and the US both had. While nations like India, where the British had a heavy handed influence, were granted a vote in the UN, individual Republics from the USSR were not.
So it became favourable to have a U.S style sphere of influence instead of a integrated nation like the USSR was.
I'm sourcing this from Peter Clarke's The Last Thousand Days of the British Empire, which talks a fair bit about the formation of the UN.
There is obviously no one reason, but the main factors were these: Technically, there were supposed to be free elections in all former German occupied countries. These countries were also given the "right to self determination and self rule". Directly annexing them would have immediately put the Soviets on horrible terms with the West and would have caused immense Western paranoia and backlash.
Secondly it would have been a bureaucratic and economic nightmare. Integrating half of Europe into one planned economy would have been an immense task.
Thirdly the Communists wanted to gain legitimacy with the people. The Eastern bloc counties had long rich histories, and to wipe all of these nations off the face of the map would have lead to massive outrage from the people in those countries. The Soviets could install governments that would do their bidding (in effect the same as their government having power) which would be much smoother and much less controversial than erasing half of Europe from the map.
TL;DR: You can't get rid of half of Europe that easy.
So most people are arguing the idea of all workers uniting in a single world wide Soviet Socialist Republic as per 1924 Constitution was pragmatically ignored as it was too unwieldy. My follow up is did this lead to an ideological change? Was it also dropped from later constitutions and formally explained as a change in the future goal of the Party/State? Or was incorporation still the theoretical future?