Central banks do not enjoy competition for their monopoly on currency. It's where they derive most of their power.
They want to have a sole control but they cannot control bitcoin so I can say it has become a threat to them. People will use bitcoin even if they will ban it because as long as there is internet everything is possible online for a person who are satisfied with the decentralized system.
It was a threat in the way that bitcoins can be used for buying drugs online and it was very hard for N.B.I. to trace the transaction so the only way to avoid this illegal transactions is to ban bitcoins from circulating the country.
Some people strongly believe that Government is the illegal transaction.
