state wants to get tax. the input and output of the money can not be controlled. also a serious competitor for bankers. also a good way to make money. countries are opposed to it. a money they can not control is troubling governments.
Tax is the most important thing for any country for their economy growth. Where as crypto are digital currency so its user doesn't pay tax to their government and this thing ultimately affects the country economy. Cryptocurrency provide many advantages and privacy to its user because of this many people use it instead of fiat currency and this thing affects the global price of that nation currency in the international market. Another reason because of which countries do not allow crypto is the association cryptocurrency with many illegal and criminal activities like anti money laundering, cyber crime etc.