Part of the problem in Wuhan is that the global health community still isn’t entirely sure what the coronavirus—2019-nCoV, more formally—actually is or how it works.
The best evidence so far is that the virus originated in bats, and transmitted first from an animal to a human in a “wet market,” a place where live animals are sold for food. And the virus has transmitted from humans at the market to other humans … but perhaps not from those humans to even more humans, and on and on.
If the virus doesn’t transmit well from person to person, that’s good news. If the virus doesn’t survive for long outside a person, or it takes a lot of virus to infect someone, or satisfy any number of other characteristics of a really pernicious epidemic—if this whole outbreak comes down to a point-source of transmission, that market in Wuhan, that’s all a relief, and it means that a quarantine is too big a hammer.
And an unpopular one, at that: The lockdown comes during New Year celebrations, when pretty much everyone wants to travel to be with family.