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.
Bitcoin is designed for global use, not just for one country. Some countries, like El Salvador, have recognized it as a legal tender, they didn't stop using their traditional currencies they’ve just added Bitcoin as an additional one to use. If a country were to use Bitcoin exclusively, they would lose control over inflation, interest rates, and the money supply. Additionally, Bitcoin is still slow for transaction (about 7 transactions per second) and volatile. Many places still don’t have reliable internet connections. It will be disadvantage for trade with bitcoin. So untill govt create the right infrastructure and laws, Bitcoin will remain an asset or a substitute, not a fully-fledged currency. I think It hasn’t reached that point yet.