There were more factors at play than just the system of government in place. The United States is a moderate region where agriculture can go on throughout most of the year; the Soviet Union, by contrast, was much further North, and much colder. You couldn't grow as much food because there wasn't enough sun and heat. Another issue is shipping. Moving things by sea costs about 1/10 of what it does to move things by land, so having a lot of coastal areas improves commerce, and the United States has lots of coastal area, as well as the Mississippi river, so there is lots of land with access to cheap shipping. The climate and access to transportation weren't the only factors, but what I'm trying to point out is that there was a lot more involved that determined the success of the Soviet Union than just the political system.