Like it or not, there is no other currency in the world that has such wide spread circulation as does the U.S. Dollar. At any given time there are more U.S. Dollars being used outside the U.S., than inside the U.S. Just as English has become, somewhat the de facto language of the world, the U.S. Dollar has become the de facto world currency. Reportedly there are over $40 billion U.S. Dollars in use in Russia. In 53 years of traveling in Canada, by RV, car or personal aircraft, I have never had a Canadian business tell me they wouldn't accept US dollars, of course the exchange rate was not favorable to me.
Try spending Canadian currency here in Florida where we have over 650,000 Canadian snowbirds with us during most of the winter, and you will most likely be refused. Thy spending some Mexican pesos or French francs if you want to get some strange looks. LOL
Why are we like that in the U.S.? No idea but it is the way it is. Many/most of our banks don't seem to know how to deal with foreign currency at the teller windows.
But with that said, I like to use the currency of the country where I am physically present, for the most part. The exchange rate is better normally. Several of my Mexican friends keep their savings and retirement funds in U.S. Dollar accounts, either in Mexican banks or in U.S. Banks. One of my Mexican friends, an architect, told me that he used US dollar accounts, because he liked to feel when he went to bed at night, he was going to have the same amount of money when he woke up the next morning.