Bitcoin has no centre to attack. There is no single organisation or person that controls Bitcoin and transactions don't go through a central clearing house there is really no way a regulator can stop people from downloading Bitcoin wallets and sending each other bitcoins. Government can only use media as a weapon.
true, but they can hinder the adoption, in many way, by stopping local exchange or closing your bank account, or saying shit like "I put you in jail if i caught you usign bitcoin", some country also do this actually
but i think it would be better for them if they embrance bitcoin, and regulate it like japan did, they would receive a better source of income via additional taxing