I think because bitcoins can not be controled by government and it can kill fiat money, if all of people on countries use bitcoins for payment method in their real life, the fiat money will be useless and it is mean the government can not become regulator and can not take taxes.
Its not just about taxes, its about the threat to financial banking systems and at the same time it is linked with the issue of being used in illicit activities. The government leaders, some of them. big people in the government has a relation with people with the banking sytems, the concept of decentralization is a threat to banks so banning it is one solution. Countries who are very strict in terms of upholding law bans bitcoin as it is used by syndicates in the black market without being traced.