William Walker,
South Korea had a pretty well developed testing infrastructure due to H1N1, which they used to their advantage in this outbreak. By pure luck, H1N1 caused this infrastructure to already be set up in their country and helped them immensely.
In comparison, the USA, and every other western country, did not need to worry about H1N1 nearly as much and did not have a need to develop this infrastructure. This disparity is what is making SK so much more prepared then many others, including the USA.
Thanks for the left wing propaganda though.
Second, as Slobo points out, population surely has a big part of it as well. Namely the fact that the USA has a much higher population than SK.