South Korea is able to institute very draconian (almost-Chinese level) measures the rest of the world, certainly including us, cannot do. They track people's exact movements via cell phone technology, surveillance cameras, credit card transactions...and hunt people down, publish the identities who have tested positive, and so on. Quarantines are actively monitored by authorities. While it would be nice to slow the growth to a manageable level like they have, there are reasons why none of the rest of the world has. I mean crap, we can't get people to even stop congregating in large groups.
They also benefit from having gone through SARS and MERS before and they dismantled their CDC-equivalent regulations in that regard.