We should keep in mind that all the transactions that are made with the bitcoin are not going through the authorities and that means that they have no way to put a tax on it. I would say that the most important reasn that makes some countries limit the bitcoins use or maybe ban it at all.
Cryptocurrencies are leading to loopholes in the current banks data about the money transactions leading to inability to track economic activities. Crypto and Cyberspace has emerged as a power in itself thus bringing a check on the activities of the so powerful governments.