Let's talk about the size of South Korea compared to the size of the United States..
South Korea: 38,691 sq. mi. ; population = about 52 million people
United States: 3.797 million sq. mi. ; population = about 329 million people
Of course it's going to be easier to contain in South Korea compared to the United States. There are so many different factors into why the United States is probably going to have a much harder time than some other countries.
That, and South Korea has water on three sides and a pretty tight border to the North.