Simply because governments like to have complete control over everything in our daily lives.
This is one reason among many reasons why the advent of cryptocurrency is a threat to majority of the developing countries. If we see the trend of growth of cryptocurrencies in the developed countries, it is swift and normal for the governments to tackle the challenges posed by it but in case of the developing countries-majority of which are already in financial crisis-the advent and implementation of crypto system in the economy can disturb the existing system whose policies have been built around fiat.