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.
This is what i also think, some country government wants to get tax because of being regulated on their country which bitcoin cannot give them as an exchange. Also this could be a potential problem on banking system, take note that bitcoin is used as a currency so there is a possibility that people will use it more than fiat currency.