This puts a hard limit on the number of currencies, and how much they'll dillute the dollar (or any other country's currency). Since you'll normally only see a single currency per government.
Cryptocurrencies on the other hand, aren't tied to any country.
There's no hard limit on the amount of them. So every time a new one shows up, they'll they'll take a piece of the cake from the already existing ones, since the space is limited by the number of people willing to invest in them and how much these people have to invest.