There are multiple reasons why South Korea was different. First, they had much better protocols for testing. They were able to rollout far more tests far faster than us, because their equivalent of the FDA didn't stall the approval of tests. By contrast, we still have a shortage of tests. This isn't because of Trump, contrary to the media's claim, but instead because of the FDA's emergency protocols for approving tests.
If the FDA had been more open to approving tests (and more efficient), then we'd probably have close to as many tests per capita as South Korea has.
Another thing that works in South Korea's favor is that their society is relatively homogeneous. Because of that, their culture is easier to organize toward collective efforts. South Koreans tend to be better about following rules in general. Granted, conformity has its downsides as well.
