Because to deliver from within China, you will need a license from Chinese government regulators. Even if you use Akamai, your content needs to be approved, before Akamai can cache it within China. Typically, Akamai serves content for customers, who are not licensed to serve from China, from the Pacific Rim - Korea, Japan etc. This is a major cause of the latency, given that China is a huge country.
However, I believe if you intend to serve your content from within China, through Akamai servers, they can facilitate setting up the license for you. This would certainly resolve all the latency.