Cash or physical currency reenforces the franchise value. Exotic Currencies like the Kazakh Tenge are not generally availible because well hwo knows? But Currencies that are physically availible like the dollar are franchises that must be defended. Imagine if you will a shortage of dollars for physical transactions? Well the void would be filled by another currency quickly. The dollar sucks the air out of the void and fills it with what is generally considered to be of value. If someone where to refuse dollars and rather EURO they would experience significant resistance.