They ban it because they can't control it. Governments are in the Control business.
No government never want something in their country which they cannot control. And in bitcoin is more likely that thing. Many government mainly ban it because bitcoin affect their economy growth. Bitcoin user doesn't pay tax their country which ultimately affect their financial status 
. And some time Bitcoin affects their country currency in both direct or indirect way. In some country like Israel they ban bitcoin because it associate with many criminal activity like kidnapping, Anti money laundering, etc. So here some reason behind ban bitcoin in some countries. 