Because the demand for dollars is 7 billion people + entities vs demand for say Chinese yuan is 1.4 billion people + chinese entities only. Basic supply and de

Because the demand for dollars is 7 billion people + entities vs demand for say Chinese yuan is 1.4 billion people + chinese entities only. Basic supply and demand. If demand for US dollar reduces (say all Asian countries band together like EU to form a megacurrency), they will stop keeping too many dollars in reserve and demand will drop, thus currency value will drop and the US will drop in GDP rankings.