Why did the U. S. and the USSR emerge from WWII stronger than other European
nations?

They were spared from having their countries blown to bits

Explanation:

Most of Europe was destroyed during WWII, and after the war they had to rebuild their whole countries, thanks to the weather in the USSR, and the ocean separating Europe from America, the US was also spared. The Axis powers were tearing through countries in Europe in an attempt to take over the whole world, destroying any and all countries in the way.