There are two big groups of reasons – one group of reasons is officially stated by almost every aforementioned country: cryptocurrencies are banned because of security issues. The other group of reasons are usually not stated officially but is largely implied. It is directly associated with cryptocurrencies being decentralized: cryptocurrencies allow people to trade directly with each other (peer to peer), cutting out the need for a middleman, which traditionally is a bank.
The anonymous nature of cryptocurrencies means that it can be used for illegal transactions - such as buying and selling drugs or weaponry. According to central bank officials, cryptocurrencies can also be used for money laundering or terrorism. The case of NHS  shows that cryptocurrencies allow hackers to request ransom payments and stay anonymous.