On interesting thing to note is the ACM World Final ranking usually doesn't correlate perfectly with perceived ranking of the school, even the ranking of its CS department.
Another point is, ACM is fun, but if you purposefully train for it, you reach the point of diminishing return quickly. Being a former Olympiad in Informatics contestant (high school equivalent of ACM), I find these kind of competition absolutely not helpful in real world programming. It does help me pass the interviews.
In China at least, ACM is perceived a way to prove oneself to potential graduate schools in America and employers. And I do know employers give very high priority to these students - even Google.
So, the reason the Chinese is good at it is mostly, this is not a hobby, but a high stake exam. It is a way to land at prestigious graduate program or job.
Why isn't the US, and why aren't top universities doing as well as we may think they would be? Because they have better options than ACM.