There are countries that are oppose to the idea of decentralization which is what the cryptocurrency is because they don't want to lose control. If you observed, the government is the one who has the right to set the prices of any product and any financial transactions should be taken into account so that government will be able to take their share of the profits through taxes. With cryptocurrency, they would not be able to monitor all the transactions because although there is transparency of the funds, the people behind those addresses are anonymous so it would be hard to pinpoint who is who.
There is also the risk of using the cryptocurrency as a mode of payment in illegal activities because bitcoin become more famous due to the silk road incident and those who are against it is using that reason why they don't want to get involved with crypto. That is just one side of crypto but it is already evolving and more companies are using it as mode of payment legally but still it would be the prerogative of the government then if they want to recognize crypto as legal or not.