The short version is that cryptocurrencies are making them nervous because they can't easily control it.
Can't easily? You can't control them at all. This is especially true if you use Bitcoin in a combination with an anonymous coin such as Monero.
why would you want to control free men?
They have been doing this for hundreds of years, why would they stop now?
Money is a tool to control the entire mankind
Money is power. This is why the rich people are just sitting on their pile of money and not doing anything rational with it.