Reference: https://en.wikipedia.org/wiki/Legality_of_bitcoin_by_country_or_territory
Governments definitely want to curb the illegal usage of cryptos but they also understand that it is impossible to enforce a blanket ban on bitcoin or any other crypto transactions due to its nature of operation. Some developed countries like Japan or Germany have studied the crypto properly and provided a framework to curb the illegal usage and promote legal usage of bitcoin. There are some countries as mentioned above, taken the easier route to ban instead of providing a proper framework. So it is incorrect to say that no countries have made bitcoin illegal, there are examples!
Those are 15 countries (count them), out from nearly 200. You could also count the ones where its regulated...
While it looks "easy" to stamp a banning law, in practice your chances from actually preventing people from using it are nearly zero. Sure, it is an annoyance, and sure, many innocent people would get imprisoned, if they don't know how to cover their tracks. But whatever it is they don't want to occur, cannot be stopped by some words on paper. They cannot shutdown Bitcoin, and they cannot block Bitcoin transactions. If two people from any of those countries privately decide to transfer coins one to another, they will never even know it ever happened. Done properly, that is, not revealing it to others, and using Tor, etc.
At least some countries are properly studying it. One of the things that come to light once you read a little, is that you cannot (really) stop it, so issuing a law against it is pointless and can only do more harm than good. And for the same reason, too much regulation is also pointless.
Bitcoin separates money from State and private institutions, this is precisely what makes it so valuable, even more than gold.