Because we are so comfortably used to the kind of system we are under right now. It seems we cannot imagine a currency which is totally free from state control. The fiat system is designed in such a way that there are a number of control mechanisms allowed. The state and the central bank inflate and deflate the monetary supply, dictate to a certain extent the currency's value, regulate inflation, and so on. It seems all these are necessary and that a fiat system will fail miserably without them. In all fairness, Bitcoin hasn't yet proven that the opposite could work. We have yet to see a country fully run by a Bitcoin-based economy.