Re: what is the main reason for india to ban cryptocurrency
The government or any third party cannot regulate the world of cryptocurrencies. This made government officials very worried. It is probable that even illegal activities are very high for the government to worry that the world of cryptocurrencies can damage the country's economy. Some countries can adapt widely to use cryptocurrencies in the country. 
They can regulate it if they want. Whether or not they can effectively enforce those regulations is another question. But even so, any types of regulations where there are penalties will surely influence people's decisions and such related to interacting with the currency.