In her new book China’s Gilded Age: The Paradox of Economic Boom and Vast Corruption, Yuen Yuen Ang presents four reasons:
1. Access money dominates.
More concretely, politicians prosper by getting things built, not by preventing things from getting built.
2. China’s political system operates on a profit-sharing model.
3. Capacity-building reforms have curtailed damaging forms of corruption.
4. Regional competition checks predatory corruption, spurs on developmental efforts, and ratchets up deals.
The book in fact presents serious data and argumentation in favor of those propositions, and thus it is significantly more useful than most of the China books you will read.