At a time where I see currencies moving away from cash to digital I think the reason is partly so that governments can control the flow of their currency more particularly for laundering. They can possibly gain more taxation from a cashless currency because cash cannot be fully traced. If i sell my car for cash to another individual the government dont get any revenue from that because it is not traceable.
So in a governments aim to control our money it will leave a big opening for bitcoin and other decentralised crypto to exist along with fiat and will allow people to "trade" anonymously with each other over a completely independent infrastructure.
This could actually be true in my opinion, now using cryptocurrency, when you do a transaction, your transaction can be traced or tracked, and as some people say, the government or some people who wanna use cryptocurrency could actually use whatever amount they had to whatever evil acts they want. But to be fair, this is only my opinion, and the government could be having a much different plan than what we think is happening.