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.
There are several factors that might influence it.
Like the volatility that you have explained. Under the pretext that they want to protect their citizens from various losses resulting from the volatility that exists in Bitcoin.
Not only that, the many negative issues about Bitcoin are also a reason for those who don't like implementing illegal regulations in their country. In this case, governments (although perhaps not all, but most) agree and consider that Bitcoin has more negative impacts than positive. They may think that Bitcoin or cryptocurrency is not real, has no backup, and cannot be held. Not only that, there is no guarantee that the price will continue like that. This is the purpose of their protection of their citizens.
Apart from that, there may also be other negative issues, namely that Bitcoin is decentralized so the government cannot control it.
However, behind that reason, there are already many citizens who have been helped thanks to Bitcoin and are able to continue their lives well with Bitcoin. Because after all, as long as we can optimize the positive side of Bitcoin then it will be very profitable for everyone.