There lots of reasons, and one is that the governments cannot benefit from it, means that they can't tax it and gain profits. Another is that it can be used to run illegal activities, mainly sell drugs and other things that aren't purchasable in the normal markets. Others says that it is a cause of scam, people lost money in this investment and the culprits are hard to find and sue them, even though crypto isn't born yet there are already scam accusations in other investment, sad but true.