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.
In the future if we can use our bitcoin without converting it to money the government will also adopt to the situation. Taxes will still be placed and taxation on cryptocurrency will also upgrade. Let us not underestimate the governments authority and resources since they are capable of many things including monitoring every transactions on your secret wallet.
They can tax when they control their bitcoin earnings. For example, a supermarket has to mention how much it sells. The arrival price of the products and the selling prices of the products are certain. He has to show his earning correctly. It has to show how much sales are done with bitcoin.
When all this information is given, he will have to pay the necessary taxes.
So someone who opens a business has to pay his tax.