Im sure you mean, why countries DONT accept bitcoin in transactions?
because at the moment only a handful of countries have recognised it as a form of payment,
but not to replace any fiat currency.
I can understand some peoples thinking that if and when bitcoin is regulated by countries maybe there is a chance it woild be accepted as legal tender but i cannot see it happening.
Countries will simply create their own regulated crypto.
in any country that recognizes bitcoin as a currency, bitcoin is only used as an alternative currency, because they still use fiat currency, because the use of bitcoin can only be done in areas that have an internet connection, while not all regions of the world are connected to an internet connection , so fiat currencies will always be used, while bitcoin will become an alternative currency.