The most accurate response to why countries other than the United States did not take a leadership role in the world after World War II is:
The United States was one of the only democracies strong enough to fight communism.
Following World War II, the United States emerged as a superpower with significant military and economic strength, while many other nations were recovering from the war's devastation. This context allowed the U.S. to play a leading role in shaping global policies, especially in the context of the Cold War and the fight against communism.