This is almost certainly getting cause and effect backwards. That is, South Korea has been testing a large number of people because there were a large number of likely cases to test, mainly due to that one cult inadvertantly spreading the coronavirus to so many people. You can see this in the number of tests that turn out positive: it's hovering somewhere around 4% in South Korea and slowly dropping now they've got through the really likely targets, but was in the high single digits earlier. Back when the CDC was only testing the most likely cases, their number was around 3%. A lot of the media reporting on this is dubious and might lead people to believe that the US had South Korea-level numbers all along and just wasn't spotting them due to not testing, but that's unlikely.