Bitcoin is a innovative thing which help people rich  in many countries it mis legal  but some countries it is declared as illegal. I think  these reasons may be cause.firstly Bitcoin doesn't pay  any tax to the government so it  may be a cause .  Secondly  the government might be thinking if bitcoin legalised they might be loosing their control on their country economic. Thirdly  bitcoin  may effect their currency  value in the international market.