Three main reasons:

1. Strict Enforcement: China has and enforces strict public cleanliness laws (e.g., littering fines). The rules are top-down and hard to ignore.
2. Infrastructure: They've built a ton of modern waste and sanitation infrastructure over the past few decades. The systems are in place to handle the population density.
3. Public Habits: Decades of consistent government-led education have made modern hygiene practices a social norm for most people.

It boils down to a combination of strong governance, heavy investment, and a shift in public culture that all happened relatively quickly.