Presumably the fiat part comes from the power to create legal tender laws or the power to collect taxes denominated in a currency, or both. Cryptocurrencies don't have either of those powers.
That is, a fiat currency's value isn't based only on objective or subjective/speculative value, but also, to an extent, on the issuing state's power to require people to use or accept it in certain transactions.
That is, a fiat currency's value isn't based only on objective or subjective/speculative value, but also, to an extent, on the issuing state's power to require people to use or accept it in certain transactions.