In many countries, it isn't illegal, however the countries that have made it illegal do so for many reasons, volatility is one of the most often cited reasons, as is energy use, concerns over destabilization, or the ease with which criminals' activities can be financed and conducted using them. Forgetting the advantage of cryptocurrency which has over the years gained popularity amongst investors globally. With technological involvement and industrialization, digital currencies are obtaining a satisfactory position over others, for example, bitcoin. By using cryptocurrency it is easy to transfer money without any involvement of banks and other financial l institutions. Inflation protection, due to inflation the value of so many currencies drops with this many people see cryptocurrency as offering protection against inflation. Bitcoin has a hard cap on the whole number of coins that will ever be minted.
You have less clue about crypto and I don't blame you because it seems you are knew, why making up your mind with this opinion of yours instead of you trying to gain some knowledge first?
Volatility isn't one of the reasons why any country put ban on Bitcoin, volatility headache belongs between Bitcoin and it's adopters, not a country, if they can put ban on Bitcoin it shows they aren't investors.
Criminal that want to finance their activities using Bitcoin or the blockchain should proceed and let's see how that ends, stop saying what you don't know, you will never gain any knowledge like this, every transaction using Bitcoin can be traced, it's transparent on the blockchain, a smart criminal will rather use Fiat instead of Bitcoin.