- it's in Chinese
- it's not blocked by the GFW
- it's faster (the GFW slows access to everything outside of it down)
- it's still accessible when China Telecom has been tweaking the routers and DNS servers (which they seem to do every week)
That is, practical reasons which lead to adoption and growth.
There are, of course, cultural specifics that have to be taken into account but those are pretty obvious if you're building a site in China for the the local market. And these local touches can only add to the site attractiveness together with being inside of the GFW. So, it's kind of obvious that a local site is going to do better than a foreign one.
Occassionally people will talk about designing the look of the site for the local market but I'm not convinced it's an issue. Good taste is good taste everywhere. For example, the iPod is just as popular here as everywhere else. A well designed and functional site in China is always going to win over the equivalent site that is outside of the GFW for the reasons above.