First is because of religion. Confucianism is a religion that worships education. Confucius is the first educator in the world, and Chinese society is a Confucian society emphasizing education.
The second is because of language. The Chinese language needs both hemispheres of the brain to process. Speaking Chinese activates and exercises both hemispheres of the brain and enables the brain to develop more cleverly.
In conclusion, the Chinese are clever not by genes but by culture. The Chinese culture makes the Chinese cleverer.