Well, it's probable that, Hitler being Hitler, his plan wouldn't have worked in reality. So long as Britain controlled the sea, a "super-Germany" would have been at a serious disadvantage relative to the historical US, which did a lot of trade by sea.
As for why the USSR didn't turn into the US, it's probably related to poor economic organization, too much resources put into the military, too little international trade, and a dependency on primary resources like oil, with all those factors and others interacting in fun ways.
The trade is a big one - one of the reasons for the recent annexation of Crimea by Russia is to give Russia another deep-water port. They have surprisingly little viable coastline given their size.
The other thing is that the US sits on very wealthy land that's in the temperate zone. Large amounts of Russia is difficult to access and the viable parts spend more of the year affected by bad weather. You can even see it in the national stereotypes - the US citizen wears a t-shirt and jeans, the Russian citizen wears an ushanka and heavy coat.
Similarly, once the US conquered the west, it was in a situation where there was no real credible military threat bordering it - it could afford to shift more defence spending to power projection rather than home defence. It's not to say political ideologies had nothing to do with the differences in success stories, just that ideology alone is not responsible.
As for why the USSR didn't turn into the US, it's probably related to poor economic organization, too much resources put into the military, too little international trade, and a dependency on primary resources like oil, with all those factors and others interacting in fun ways.