1) Why go national, when you can go international - Bitcoin
2) A national cryptocurrency (I am assuming you are thinking of an INR backed, decentralized, crypto) would be subject to the same problems as fiat currencies - capital control and debasing of value
3) If you think of it, decentralizing a national currency might not offer any additional value. You already have e-money and NEFT/IMPS/RTGS which can transfer money quickly and cheaply. And the PM is doing a pretty good job in making sure that people have bank accounts.