Because of the volatility of the price of cryptocurrency right now. It goes up and down way too fast that make a lot of people rich and a lot of others go broke. This fact makes cryptos themselves very questionable about whether they could stay for a long time.
Government of a lot of countries really get afraid of things that are not beneficial and not trustable that could affect their economies.
Volatility should be something every single person investing in the space should be able to understand and handle all by themselves, and should not be a criterion for the government to want to ban cryptocurrencies. Sure, we may say the main reason is that it is an uncontrolled space and that gives room for any manipulations, but as time goes on, and with some lenient form of regulation, I want to believe all these would change.