That is because China took drastic measures to control the spread of the virus. Many of these interventions were existing systems and cultural norms that were already in place due to the previous SARS epidemic. The US, for example, gave up on contact tracking fairly early while China had 1500 teams of at least 5 people doing contact tracking(and are still doing it)

If R0 is in the range of 2-3 the interventions need to reduce transmission by 75% to 85% to get R0 values down to 0.5. At that rate exponential decay starts and the math works the same in the other direction.

The China or South Korea data isn’t especially comforting when you look at the scale of interventions required to bend the curve.

https://www.businessinsider.com/chinas-coronavirus-quarantin...