Because Fiat currency is directly tied to the country, and the economy.  Cryptocoins are universal and not tied to anything.  Also Cryptocurrencies are still the wild west.  Far to many people out hacking/stealing ect.  And Wallets not as easy to use as handing someone a $20 bill, and 1 wallet update gone wrong, or every unbacked up wallet that occurs on a device that crashes add a pile of things that will cause the majority from accepting it.

As far as things like Anonymity and taxation.  Those are far easier to control with a cryptocurrency.  Fiat is fairly untraceable.  So I don't buy those as being reasons at all.