I would say it's not that unexpected though. The first person fell ill in China in the beginning of December but the outbreak was reported 30 days later only and that's when the first precautionary actions started - and to be honest, outside of China, there weren't many precautionary measures except temperature scanners and then later stopping flights from/to Hubei province. Considering the fact that the virus is not extremely fatal and asymptomatic people can also spread it, it's quite likely that it was brought to numerous countries well before the Chinese cities were quarantined and travel restrictions were put into place. Add incubation period on top of that as well as the fact that about 80% of cases tend to have mild symptoms, and it's very likely that the spread simply went unnoticed until now. Now that more people are starting to get sick due to human-to-human transmission in numerous countries, more people also experience more serious symptoms and seek medical help, so more aggressive testing is being carried out instead of only testing those who have symptoms and have traveled to China, resulting in a sharp increase in total numbers. I think once other countries start doing the same, we'll start seeing a lot more confirmed cases there too.
Also some interesting facts about the outbreak in South Korea - most of those cases are linked to that weirdo religious cult (the leader of which - I kid you not - claims to be the second coming of Jesus and says the virus is "the test of faith" and the work of the devil hellbent on stopping the rapid growth of the cult). So anyway, the cases in the cult are pretty much all traced to one woman who got symptoms but refused to be tested and sneaked out of the hospital at least four times to attend services of the cult until she finally got tested and confirmed to have the virus. In addition to that, as more members of the cult started getting sick, they were still being encouraged to continue their "evangelical work" and to deny their affiliation to the church if public health officials asked. So all in all, a bunch of inconsiderate religious nutjobs are pretty much at fault for the outbreak in South Korea.