It's the trajectory. It's growing exponentially, and there are more and more little outbreaks outside the country, as well as within China. I wouldn't expect the next country to fall to be the U.S. or Germany, but somewhere like Thailand or Indonesia, at which point it will get all that much harder to control.
Look, I still think there's a chance this won't become a pandemic, but the longer it keeps going in China, the riskier it gets. Even ignoring the potential deaths, the economic risk of this grows day by day.