TL;DR Kennan and other US policymakers were concerned with Soviet foot stomping, which would go against US interests. Soviet foot stomping would often take the shape as Communist revolutions, hence the Domino Theory.
Since no one else has answered your question, I'll give it a shot even though I'm not as well-read on the subject as I would like.
You're quite right that with Stalin's "Socialism in One Country", the idea of a worldwide communist revolution died as a mainstream idea. However, a man named George Kennan was less concerned about worldwide communist revolutions than with general "revolutions/foot stomping" caused by the USSR.
In his "X-Article" or "The Sources of Soviet Conduct" (1947), which would be the basis of the Containment theory and later the Domino Theory, Kennan essentially said this:
Part I
Russia, given it's history of invaders (pre-WWII), and Stalin's own personal nature, is paranoid about foreign influence/intervention.
Since authoritarianism is how the Soviet's style of Communism is done, they are going to consider any criticism of their rule as hostile. They have already gone to great lengths to minimalize any domestic criticism to their rule.
Part II
Communism and Capitalism are viewed as inherently antagonistic. Stalin may have put aside the idea of worldwide revolution, but it is arguable that he did this for practical, and not ideological purposes. He could have been more focused on securing his rise to power and/or focused on development domestically for the time being to make sure the USSR can fight back any foreign influence/intervention.
Given all of the points so far, the Soviets are going to make sure they have "friendly" countries neighboring them. Countries that bend the toe to the Soviets and are Communist.
Part III
The USSR's recent experience in WWII has made them even more paranoid and distrustful of Capitalism and the West than they were before.
The USSR has made some economic success, but not to the degree that was promised. They may hide the public's focus on this success by turning their attention outward, away from criticism of their absolute authority. Furthermore, if they do make economic success, then Stalin or other Soviet leaders will be free to focus on other things, such as re-vitalizing the Marxist idea of Communist Revolution.
This leads to Containment Policy. I highly encourage you to read Article X on your own if you're interested. I only skimmed it as it's been a while since I've read it for my Politics of Russia class.