To me, historians call it the American Century because the United States was central to everything that happened in the century after WWI (and the US even had a large part in that).
It was US might that was key to deciding World War II. The competition between the US and the USSR dominated the world from the end of WWII up to around 1990. After that, the US was the only superpower for the rest of the century.
In addition, the US was the economic powerhouse of the world after WWII. The US economy boomed, and that pulled economies like that of Japan and, later, South Korea, with it.
So the US dominated the 20th century in terms of military and economic power.