I think the top most reason is government is unable to control bitcoin when it is easy for the government to control the banking system.So, if they make it legal then they will lose their control on the economy and they afraid of it.
The government as a financial authority certainly does not want bitcoin to disrupt the financial system that has been running so far, both domestic transactions and transactions between countries. Banking as a financial operator that is connected throughout the world, makes the government easily withdraw taxes, oversee money laundering and financial system monitoring for budget or fiscal analysis. Because bitcoin users are autonomous, it's difficult to track bitcoin transactions.