I was just wondering since it wasnt the only country with nuclear weapons or a space program, the british and the french did as well. Also, they could not keep Romania or Yugoslavia in their sphere of influence and they were at their doorstep.
The short answer is their military and ideological differences to the US and her allies.
Long answer: The USSR presented a serious challenge to the US post WWII. One must first realize that post WWII the US was economically and militarily superior to the entirety of the world. The US largely achieved this position by providing a "carrot" (or a bribe, if you will) to the rest of the world. The US provided large amounts of funds and support to both Japan and all of Europe (including Germany, albeit West Germany), almost forcing these countries and regions to support and accept US hegemony (supremacy) in the world and the newly established UN. The USSR was a power that acquired nuclear weapons (not long after the US; allowing for a relative balance of power), had a completely contrary ideology (which countered essentially all primary US interests), and had a military threatening to the US. While countries like the UK, France, and West Germany were weak (following WWII) the US took advantage of this to allow for a US friendly international environment. Essentially, the US aimed (and still continues) to build a world order aligning the interests of the world to that of the US through the carrot and stick method (if you accept US hegemony you are rewarded, if you reject it you are punished) . The USSR was fundamentally divergent of US interests, actively aiming to create their own allies and establish regional supremacy with the eventual goal of global hegemony. The combination of this divergent ambition, nuclear weapons to back up this ambition, and military power to project this ambition presented a serious problem to the US.
One great source/book to support this comment is After Victory: Institutions, Strategic Restraints, And the Rebuilding of Order After Major Wars, written by Political Scientist G. John Ikenberry. If you would like, PM me and I could provide you with my personal notes on this subject and the book I referenced; otherwise, let me know if you have any further questions, and if I am qualified I'd be more than happy to answer them!