If this Wuhan lab theory is correct, I think the wet markets play a huge role in the outbreak. They place a huge number of diverse, weakened, living animal in a relatively confined area. This seems to give an ideal breeding ground for a leaked virus - which perhaps can be incubated in a host species found at the wet market. Either the animals are in too poor of condition to notice the infection or they're a natural reservoir - eliciting no symptoms.
My understanding is most other countries don't have wet markets like China does. Even if a virus escapes, it may not have access to the hosts it needs truly become problematic.