It maybe the last thing a government want, ever since the ancient kings who rules the world control the flow of money to control its people and it never changed up to now. Today if a government adopt blockchain technology to create their own cryptocurrency they are practically letting loose control of money and therefore the people for sure they dont like that to happen right? So if they create one for sure it will be like another fiat or digital fiat. One obvious reason they wouldn't fully adopt blockchain technology is that it will kill the whole banking industry of the country.
There is a news that China is going to start their own cryptocurrency (link below) but I think they don't know yet how it can be done, no details in the news.
But for the sake of argument, let say many countries developed their own derivatives of Bitcoin, for sure their crypto will be pegged to Bitcoin (like the USD of all fiats) and if that happens Bitcoin will still rule....

https://coinidol.com/china-creates-its-own-cryptocurrency/