Your wife living in the US is the main reason why her parents were rejected.
She's a strong tie to the US for them.
Things like well paid jobs, house and investment properties, other assets and close family in China may help with next application. I would wait for a while before retrying.
Did her parents have strong ties to China?
Losing visa fees and transportation / accomodation fees is understandably annoying, but travel to the US is expensive too!