Well from what I've observed, nations/countries themselves haven't introduced bans on bitcoins except Ecuador I think but the rest has to do with central banks in those countries who have seen bitcoins as a threat and have therefore issued bans against its dealings and usage. In a country like Ecuador the ban makes the most sense of any ban, as they are building a national electronic cash system, so they feel the need to protect their new currency from something clearly superior. A decentralized currency of finite production that cannot be manipulated by governments or banks might dim the lights on their new program in the eyes of the public, which offers none of those benefits.