1 Developing nations need surplus stocks of foreign currency. These countries fear with the legalization and permission of trading in crypto ; there may be uncontrolled outflow of currency resulting in very tight foreign currency resources available.
2. Terror funding ; black marketing; money laundering becomes easy. Many nations like India are suffering from the threat of islamic terrorists. If their funding increases the attacks will increase.
3 Many citizens of a country having black money will easily divert their funds to safe heavens like Switzerland.
So India like many other nations is having fear for it. Though Indian government wants the currency to be digitalized . May be they are doing any research work for allowing crypto transactions legal because they also know it's benefits.