What is that universal reason that applies to all country that do and will ever exist, and makes you totally convinced that "nowhere" would do it?
Countries like to be in control of their currency.
And yet there are a bunch of countries using the Euro, and another bunch of countries which either us the USD or have pegged their currency to it. I would like to print all the money that I need, but since I have little power, nobody will let me do that.
That being said, I would presume that "nowhere in the world" and "never in the history of mankind" are unlikely useful for this purpose, but for "freedom of speech" sake... you are the master of your keyboard.
Bitcoin would not be improved by being made legal tender anywhere in the world, and nowhere would do it.
They would not need to be doing it to improve bitcoin, all it would take is for some jurisdiction which has a large population of bitcoin users to add bitcoins as a legal tender option to acquiesce to the convenience of the residents, they would not even have to make it the only legal tender. Legal tender just means you can use it to pay things like taxes and legal judgments.