Speaking why the US dollar is very dominant should look back to history how the US dollar can become a world exchange rate (
https://www.thebalance.com/bretton-woods-system-and-1944-agreement-3306133)
According to the article above, I conclude that the US dollar has become dominant because:
1. The origin of the dollar can be a world exchange rate, because the United States gold reserves are the largest, so the developed countries of the United States alliance in the second world war agreed to control the local currency with the dollar.
2. The United States is the winner of World War II. The US has a stake in recovery and investment in many countries affected by the war
3. As the winner of the world war, of course, the US military power is the largest, so that the security conditions are relatively more stable so that the economic and political climate in America is more conducive
4. the total value of sales of all goods and services in the United States is still the largest (GDP) so that currency traffic between countries is very high so that the USD is still needed.