So far government has no power over bitcoins yet as it is on a decentralized platform and i feel it will gain more market power globally even though few countries banned them.. just like in the past when share market was introduced in the past it took lot of time for it be accepted by the public like wise its normal in the case of bitcoins aswell hence i don't think that the government has to make it illegal but it illegal in some countries to buy or trade on bitcoins as it is banned
As said, the governments have no solid reasons to make Bitcoin illegal. Bitcoin transactions cannot be banned as well because it is decentralized which makes it under no control of any centralized or single authoritative body. Bitcoin will be rather legalized with time as its adoption grows gradually. The people will tend to understand and appreciate the principles and benefits of Bitcoin and this is where the governments will stand with no choice than to legalize it.