Crypocurrencies are banned because they are an independent store of value, which is not submitted to fiat money inflation.
I think they are afraid about the illegal used of bitcoins. The reason of Some countries who banned bitcoin is the continues of illegal trading and transactions in black market with use of bitcoin as a payment because we all know that bitcoin is hard to detect. I think they dont want that to happen to their country thats why they dont allow it.
I think that every country has their own rules and regulation, they have their own policy which they have to follow. I think that most of the countries are not allowing their people to use bitcoin as they consider it not good for their economy, but I am sure that they will very soon change their decision about bitcoin and will let their people to start using bitcoin.