The bad press that BTC and alt coin get related to them being in a bubble state, ICO scams, and lack of knowledge will make people scared of it. The fact that some people uses them for illegal activities make them being associated, and assumed illegal, by people that don't understand the technology and the projects.
Apart from that, some governments do treat them as illegal, and depending on your country taxation rules, and the way you trace crypto, might get you in trouble if you are not aware of the details