The major reason for this is World War II.  In that war, all of the other major powers in the world were left devastated.  Their economies were severely weakened and they could not wield the kind of power that they had before.  This meant that the US was, almost by default, the richest and most powerful country in the world.

In addition, the Cold War forced the US into a position as the leader of the free world.  The US could not retreat into isolation as it had after WWI.  Instead, it had to take the lead in opposing the Soviet Union and communist expansion.

The devastation of WWII and the needs of the Cold War made the US the most powerful country in the world and a global leader.