The global economy is nearly 80 trillion USD per year. Payments using any type of cryptocurrency as a medium of payment is a tiny fraction of this total.
What are some of the reasons preventing cryptocurrency from being more successful as a medium of payment?
Probably, the problem is the market volatility of cryptocurrencies. Fiat as a medium is having a stable market value which us an important aspect in order to promote equality. So since the market value of most cryptos are lacking stability, making it a medium of payment won't be as effective as the functionality of usd and other regular currencies.