because it is out control of central bank and not depeond on $ or gold or oil
There are lot of reason why they banned bitcoin. One of the reason why they banned it is because they can control and we all know that bitcoin is decentralized where no one can control it. In addition to that, we all know that bitcoin was tax free so that they banned because they could not get any tax.
Some governments ban Bitcoin because their own national currency gets the value due to excessive use of Bitcoins. Moreover the other reasons might include the increasing rate of crimes in term of the bitcoins because any transaction could be done with complete security and also that the information remains non traceable. Moreover banning the bitcoins in specific countries is just to give a way for another price increment.