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.
It is not about the tax, there are many other things that stop a government to accept cryptocurrency as legal. It is not possible to stop people from investing in crypto and finally they will accept crypto as legal and regular currency. There is no reason for banning crypto because the government has nothing to do with crypto. Now they can also track illegal use of crypto.
Sometimes illegal activities is the reason of one country to ban crypto because they cannot control the transactions over it. It has something to do with tax but they can see how there people help by it to earn and help in the economy. It is not visible to the government that their people actually pay their tax by other means because every transaction has a fee where the private companies will pay tax from their services.