Now when talkinng about about reserves, the first thing that should pop up in our minds is "stability." That’s why governments keep gold, and foreign currencies like dollars, and other assets that don’t easily swing in price. Now if a nation decides to bring Bitcoin as a national reserve it would immediately face some set of problems. One of which is "volatility," coz Bitcoin’s price tends to swing from time to time. Now Imagine government want to pay for foodstuff imports or maybe repay international loans, only to find that Bitcoin has dropped in price... That kind of shock could further bring about further crisis instead of solving one.
Every nation that considers using bitcoin as her reserve already knows that bitcoin is a volatile asset and so would make plans on how to leverage volatility to her advantage. with the USDT, stability is guaranteed but there is no guarantee that more profitability can ever happen. most of the nation that consider using bitcoin as her reserve looks at it as an option hat serves as an edge against inflation which is what the fiat or gold can not really do as much as bitcoin can do.
Gold is not a perfect invention and certainly not a perfect reserve currency. the fact that gold is a physical asset that can be destroyed if an emergency hits the country that is using it as her reserve is an issue with it but with bitcoin, such is far from the case.