The U.S. dollar is currently accepted as the world’s reserve currency, but it hasn’t always been this way. Reserve currencies change depending on macroeconomic trends: typically, the reserve currency belongs to the world’s most stable and influential economy.
As HowMuch.net notes, the U.S. dollar has been the official reserve currency since the end of World War II, when the world’s powers agreed to implement the Bretton Woods System, officially setting the U.S. dollar as the anchor currency that could be exchanged at a fixed rate for gold.