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.
He is simply just trying to protect that their USD that is already getting week, and bitcoin should even be the least of his problem now, because generally, it is even strengthening the USD as against what he thinks because when it comes to pairing of currencies for trading, I think people even pair more of USD which means they get more increased fiat in USD which is still in a way protecting the value of the said currency.
There are more things at hand that trump needs to face and it is not the cryptocurrency that is the issue now, but the debt that he needs to settle which is getting too much and some trade wars that he needs to settle with china. That is where his focus should all be in now and not cryptocurrency that is doing nothing but to make their economy better.