Nice post
There is a clear logic behind this decision, we have to understand that rise of one legal tender can easily degrade the one which is already in use, when more and more people will start using cryptos as legal tender the value of already in use legal tender will decrease. For example INR i.e India's legal tender which is already facing many difficulties will see the downfall when currency like Bitcoin will rise.
Another point to note is that it's not banned, its just not a legal tender yet .