why bitcoin is still illegal in many countries of world 
Is there any solid reason behind this ??/
Is there any solid reason behind this ??/The prohibition of bitcoins by some states is a problem of those states. Bitcoin is a legal system. States illegally define everything they can not control.
But the bitcoin can not be removed from the ban.
Bitcoin can't be banned. That is the problem. It is not a physical asset, and therefore the government can't prevent its entry or exit from their jurisdiction. Also, unless the coins are converted to fiat, the authorities can't track them.