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.
Because it is a digital currency than the transaction through online, no one can cotrolled it. It can be used for the bad transactions because no one can trace it.
Yes no one can trace it at this time, because government did not already discover a machine detector for a digital currency. And i think it is not good also for our ecinomics.