its because they want to stop manipulating the price of digital currency and avoid terrorist to use it.
No even they dont want to manipulate the prices of bitcoins, they can never do it. Bitcoins are totally decentralized and if some country would legalized bitcoins, even though they will be unable to control such coins. These coins cant be controlled. And the main reason behind not regulating bitcoins is just this one. Governments want control, but bitcoins disallow that so deadlock continues.