Look at the top 100 Chinese websites here
China - Alexa Top 100�Sites
The majority of these giants are dot coms, they will simply not rebrand their presences to dot cns.
Sina (新浪) uses .cn has it's own reason. It is a global web company having portals of sina.com.tw, sina.com.hk, sina.com.cn etc targeting different regions of the world. The com is set up for North America region. Of course they HAVE TO use .cn for the Mainland region.
If you live in China, you will see that the vast majority of advertisements are in Chinese.com format. You will also find that some advertised Chinese.coms actually having no websites set up. People just love the "Chinese.com" format; it's like a fashion (some people blindly copy other people's ad format).
卡西欧.com (CASIO)