Why in some countries Bitcoin is banned is the accepting of Bitcoin by websites and that are illegal .
because due to use of bitcoin there is flow of money from one country to another country without tax and order . to controll over thus type of flow of money in form of btc some of the countries declared bitcoun as illegal activity .
Not only in some countries I think max number of countries government treat Bitcoin as an illegal currency. The main reason is the government will not get tax from this transaction. And the Govt will not able to track these transactions. So only many countries banned this Bitcoin.