Bitcoin can't be a national currency because govs won't let it be.
We're still in the beginning of bitcoin development, still on scale debate to make bitcoin could afford so many transaction per second and we shouldn't pay a higher fees. Another factor that may a big problem is bitcoin volatile, a country can't adopt a currency which always fluctuate, even third world countries in Africa using US Dollars as their legal currency.
this is so true, already bitcoin has created a threat in the mind of government and others as bitcoins has a transaction transparency which cause no idea to the government about the online transactions so that they won't keep an check on these and neither they get to know about it. Not even these bitcoin is free from any transaction fees which make government lose the opportunity to imposed tax on the people's on every transactions. So this is what trouble them more. So I don't think so that this will be going to take place ever in future that bitcoin is accepted as a national currency in a country.