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.
Yeah, world is changing with the passage of time with new inventions and innovations and use of those technologies are in our benefit but the fact is that it is we to do something in order to enrich ourselves with the opportunities we have. Government can use cryptocurrencies for their benefits as well but actually they are thinking that it will disrupt banking system which is actually not, they can handle both if they want.