Cash and digital currency are two different things. There are still people all over the world who don't know about bitcoin also every country have its own currency and its own regulatory to keep its cash flowing.
There is also something not many are talking about right now. Is Bitcoin really a currency? I think right now, Bitcoin is more of an asset (like gold) than a currency, as the possibilities to buy goods or services with Bitcoin are still very rare. People need to be aware of that. There is also the fees attached to the transactions which prevents Bitcoin from being a real currency
today. It might change in the future.
It is an asset though, as I am personally quite pessimistic about the currency of my country (EUR), therefore holding Bitcoin is like holding gold, and obviously at a much better interest rate right now, that's why more and more people get interested into cryptos as a whole.