It's coz this virus is "novel". Basically no one have resistance to it so it's possible for it to cut swathes of people unchecked, especially with its long incubation period where it can spread undetected.

There's no vaccine for it like what is released every flu season. If it hits you you better hope you get support to help your body fight it off, which could be hard once hospitals get overcrowded once everyone calls sick.