they are afraid because if the government suddenly prohibits all transactions then we will lose a lot of coins. our coin remains in the wallet becomes a useless item can not be sold. and the government has full power to do so. they are banning because they can not withdraw taxes from bitcoin users.
As i write i don't know any government around the world that has been able to successfully and completely banned bitcoin because there is no system in place to ensure that except bans on ICO's and exchanges from operating in cryptocurrencies and that doesn't stop users of cryptocurrencies in those countries from trading them on an international platform so there is no need to fear the governments in my opinion because even if they ban all exchanges around the world we can still buy and sell bitcoin on a peer to peer basis and meet in person without any third-party interference.