Leaders like Trump think they can control the flow and volatility of Crypto currencies just be sending out one tweet. In a way they are scared about how this new form of digital money is giving power back into the hands of the general public.
The mere existence of crypto currencies is a threat to governments and banks because they have a feeling they will lose control over the population and how and where they spend their money. Banks regulate our funds according to their needs and with the emergence of crypto currencies, controlling the spending power of the public has become an issue for banks.