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.
Yeah. That's why the states do not like bitcoin. Because they can not get tax from the money earned with bitcoin. Only when they are converted into cash and transferred to the bank account.
But in the future we can use bitcoin without converting it to money. Then they will not be able to follow in any way.