Maybe because it is decentralized and a lot of transactions, including illegal ones, are out of their control. Scams and illegal matters may harm their people specifically those who do not have enough knowledge. Banning it in their country will increase security and protection for its people.
Yes, it is also a reason make the government consider to using Bitcoin and accept it is legally. However, the criminal can still using other payment method or cryptocurrency without accept of the government. Because the crimininal are not stupid to show public their transaction 