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.
Well, that fact with the government not having the power in their hands as usual is one major threat to them and one of the reasons they have been trying as much as possible to put a huge strong hold in the space. It is a deregulated or simply say decentralized space and that simply goes against the normal form of government ways of doing things with regulation. Apparently, we should expect there is no way it wont end up this way until either one would have to succumb, or both will just have to go hand in hand to make everyone happy.