To be honest its not like that they cant control it, off course it is they have all the rights to amend things if they are not good for the nation. This is what they are capable of doing, no matter what how helpful, blessingful it is for the people but if it is gonna eat the nation as whole then they, the government can rip the thing off.
The only thing that is keeping them away from this doing is they dont want to creat outrageous impression out of it. They know very well that this tech is new and can be helpful in many things. So I am pretty sure that they are looking for middle ways to come out.
You are right when you say that they can regulate cryptocurrencies to a certain point but I do not think they can control it, let's suppose that they forbid holding cryptocurrencies, there will be many people that are going to avoid cryptocurrencies from that point on but there is also going to be a significant amount of people that are not going to comply, what are they going to do at that point? Are they going to put people into jail, even if they do that how are they going to find them? How are they going to block to transference of bitcoin in a network they cannot control?
Governments don't do things that are good for the nation, maybe there are couple governments in whole world that do that, but most of them are working just for their own interest. They are corrupted, to much, only way to escape from this corruption is to change entire system.
Believe it or not, they will put people in jail (if that didn't happen already), they will go even further, some governments are capable for big evil. You can see that from the past.
Only thing that gives me hope are ordinary people across the world, blockchain give us a glance of what is better, people are awaking from some dream with crypto. We are more opened for better things, not what governments tell us its better, what really is better. Just decentralized system can help us to have fair world, and we must have go in that direction.