Firstly, there is no backing to them. While other fiat currencies moved away from gold standards and the like some time ago, they do have backing - the labour of the citizens of the country. That has a definite value. I dont see that with cryptocurrencies - like diamonds, they only have value because people think they do.
Secondly, you cant spend them. A handful of places accept bitcoin, but very, very few. Even less accept other types. So if I want to buy bread, pay rent or pay for a taxi, I have to convert it to local currency. This entails extra costs, and extra risk as the conversion rates are volatile.
As a tertiary one, bitcoin especially causes problems at a time we are trying to reduce wasted energy. Massive server farms are using astonishing amounts of power to create these coins. Thats not sustainable, in both meanings of the word.