For half a century, the American dollar has been the reserve currency of the world. Seventy percent of all currency reserves are in American dollars.
This has a lot to do with the fact that oil, the most important commodity traded in the world, is mostly priced in U.S. dollars. The majority of countries, being oil importers, have to buy their oil in U.S. dollars. This, together with related economic considerations, encourages them keep most of their foreign currency in dollars.