I think one of the reason is that America is one of the biggest economy and she gives aid more to countries than other countries and so those countries have to exchange what they have or want to buy in American $. America is the biggest economy that speak English and so it sounds convincing that their currency stands out to be used as medium of exchange and transaction in global market. Euro is just a recent currency and the euro nations are still divided about euro and pounds, so that still gives $ an edge. Yan is not popular globally unlike $ whose owners (America) have subjugated and subdue smaller financially dependent countries.
America maybe greedy and obvious with such kind of things but almost all the western countries are doing it with African countries. They will say American is cutting off the benefits they give to other countries, the health care and contribution to word health organization but China, Russia, France, US, and UK are doing that to almost what America is doing to them to other African countries but you people will rather focus and hate on America for trying to minimize their spending.
It mustn't a most any country have to depend on another country for help. If something is not working, try to improvise and work it out. It's even better you don't depend on any one so by the time they don't find you beneficial to them, it doesn't affects you. Trump feels like the United State is wasting money on some of this things, some times this drugs and aid doesn't get to proper place, it's not like they don't do their own research about distribution to avoid corruption and stealing of items for sick people.