Think of it from the point of view of government officials. Bitcoin is scary because it's anonymous. They don't know what the bitcoin users are paying for. This means that if they legalize it in their country, it's possible that they've legalized a currency that can be used for drug trades, terrorism, weapons, etc.
This is dangerous because the government is not supposed to scare the citizens. Besides those reasons, I don't think there's a strong enough reason to ban bitcoin.
They ban also because they want to protect citizens from the most risky assets comparing to the regulated stocks market. Most government is not even understand the technology behind bitcoin good enough to regulate and they ban what they can't control.