In part, presumably, because Ebola appearing in Atlanta would be extraordinarily unusual. East Asia has a history of novel respiratory diseases, the same way that other parts of the world have a history of mosquito-borne blood diseases.
Put another way: everything we know so far is circumstantial, and some pieces of circumstantial evidence are (or would be) stronger than others. An Ebola outbreak next to a BSL 4 lab in the United States would be a significantly stronger piece of circumstantial evidence than a coronavirus outbreak in a transportation hub city in East Asia.