I mean BoA & SM did give China kinda a trial, but we all see how that ended up. I think the reasons that labels gave up on China:
1- The copyright infringement there is way too complicated to handle.
2- China has the most population on earth, therefore they don't really need to 'expand' their music to other countries, say, if only 1/4 of Chinese population buy Jay Chou's album, he would sell over 3 millions copies , 3 times more than BoA's sales in her peak all over the world.
China is a big market to conquer for any label company, but it's also the most challenging one. China's music market is just closed, they are too content with their music to accept any other music in different language.
Unfortunately, BoA doesn't sound good when she sings Chinese, imo. Hence, I'm kinda glad that she dropped China