Most countries fear that bitcoin would be mainly used for illegal products like drugs and for money laundering.They think that bitcoin usage would reduce their currency usage.They fear of losing their revenues which they get from taxes.In bangladesh,if anyone found using bitcoins, they would be jailed.But whatever measures taken, bitcoin cannot be fully banned since its not possible to ban internet.
There are only just a few countries that are officially supporting Bitcoin. Most other countries are just ignoring it, and there are a few countries that don't want it to be used at all. Even in China, which is now the center of the Bitcoin mining industry; The Chinese government allows people to use Bitcoin but it has banned banks and hedge funds from using it.
In many countries, governments want to control the use of money very strictly. They know that Bitcoin can be used for tax evasion and also for money laundering, but some just don't like there to be such an anonymous form of payment. There are even some governments that know that the future of money is digital and, when they issue their own digital currency in the future, they don't want it to be competing with Bitcoin.