I feel Bitcoin won't be only a currency for a country. Because governments don't want to lose the control from their native currency. Bitcoin is uncomfortable; cryptocurrency means no one would have control over that. On the other hand, transaction fees are another prevention. We know a large number of the population don't know about Bitcoin and how it does work. Those who are illiterate can't handle Bitcoin anyway. So we must have physical notes for our daily life. Which isn't possible; we can't imagine what will happen if Bitcoin becomes a currency for a country.
That’s probably the reason why bitcoin until now has not become a country’s currency, except for El Salvador who made it as an official currency.
It requires a mass awareness and adoption, and most likely mass education so that all the citizens living in a country will be informed on how to use bitcoin properly. However, if a single country really aims to use bitcoin in their country, it’s actually possible but it takes a lot of preparation prior to that so that the people won’t get confused and stay complaining in the long run.