P.S. The answer to questions like this is always part logic or biology, but mostly culture.
For example, in the U.K. before 1907 if a man's wife died, it was illegal for him to marry his wife's sister. (His "Deceased wife's sister.") Why? There's no genetic or biological logic to it. The sister isn't a closer relation to him. It was based on culture and religion. It just "wasn't right." Why not? Because it just wasn't.
Then in 1907 a law was passed and it became legal.
As for your question, I would hazard a guess but it's just a guess. My impression is that in Chinese culture, people FEEL closer to their "extended family" than they do in the United States, and therefore--regardless of genetics--marriage between cousins is more likely to FEEL "wrong."