There are many reasons :
1. Transparancy { A lot of the more corrupt governments cannot accept a currency, where every transaction is published openly for EVERYONE to see on the Blockchain }
2. Governments needs to manipulate currencies { Reserve banks determine how stong your currency can be, according to their moneratary policies } The Core Bitcoin protocol is fixed.
3. Anonymity {Most countries does not have the abilities to identify Bitcoin users} and this pose as a threat for security { Terrorism funding and Money Laundering and Crime etc.. }
4. Some governments want to protect the consumers from fraud / theft etc.. { It's not easy to track and retrieve stolen/hacked coins }
5. Other governments just want to protect the value of their own currency. { Money exchange and remittance }
6. Do not forget, that huge fiat financial organizations are backing politicians { Who pay for the election campaigns? }
There you have some of the reasons.