That's because South Korea rolled out massive testing early, so they caught it before more extreme measures were necessary. On top of that, Koreans did a good job of voluntarily complying with social distancing measures.
> Everyone who is infected in South Korea goes into isolation in government shelters, and phones and credit card data are used to trace their prior movements and find their contacts. Where they walked before they fell ill is broadcast to the cellphones of everyone who was nearby.
> Anyone even potentially exposed is quarantined at home; a GPS app tells the police if that person goes outside. The fine for doing so is $8,000.
https://www.thenation.com/article/world/coronavirus-south-ko...
And from the article:
> Everyone who is infected in South Korea goes into isolation in government shelters, and phones and credit card data are used to trace their prior movements and find their contacts. Where they walked before they fell ill is broadcast to the cellphones of everyone who was nearby.
> Anyone even potentially exposed is quarantined at home; a GPS app tells the police if that person goes outside. The fine for doing so is $8,000.