First of all they don't know how to demand tax from bitcoin and also the fear of they economy being destroy by bitcoin is another reason. And also the government are afraid of bitcoin because they can't control activities since bitcoin is a decentralized currency and not under any central bank of any country, the government are afraid that it is easy to launder money out of the country using bitcoin. So many reasons leads to some country ban on bitcoin.
Implementation of taxes is the primary due to which each and every government and in case of bit coin they did not know the way that how they will implement taxes on bit coin which is decentralized crypto and virtual currency and it is very hard to put taxes on bit coin.