Because Bitcoin currently has to be converted to a country-supported currency to be spent. If you could buy everything directly with Bitcoin then the Bitcoin-to-currency chokepoints would not work (and it is possible that some volume of Bitcoin already escapes such control points). I'm starting to think of cryptocurrencies not as currencies or like gold but as a replacement for banks and exchanges and SWIFT. Central bank supported cryptocurrencies may encourage much wider acceptance of them as a direct person-to-person/business-to-business/person-to-business/government-to-person method of exchange, bypassing banks. Of course government approved cryptocurrencies may also be designed to track all transactions complete with identity of the participants.