Cryptocurrencies are a new concept of money. They aim to substitute the traditional monetary system we have. It seems hard for them to go there because of the strong disapproval from central banks and governments they are facing. Why are cryptos not likeable by the governments? Simple. Cryptocurrencies are created through the mining process of their algorithms. They are decentralized units, so central banks and other authorities which are controlling fiat money, have zero control on cryptos.
In contrast with fiat money, the amount of virtual money is limited. There is no way of issuing an infinite number of digital tokens. Bitcoin, for example, can be mined maximum at 21 million Bitcoins. This is the reason why cryptocurrencies are compared with commodities which have a limited amount disponible, and not with US dollars which can be issued by the federal reserve.