Because of the fear of the unknown and the inherent risk since crypto is still in infancy. Government like to be everything within their control .
They fear bitcoin precisely because they know very well the size and the strength that it can reach, bitcoin is not really an unknown to them, they fear the almost limitless potential that bitcoin has and that is going to be impossible to control if it's allowed to flourish, but they are finding that it is almost impossible for them to control it and that is why they are banning it.