One reason: It doesn’t have WiFi, while grey market versions of the phone do.
Apple’s Chinese partner, China Unicom, tells the WSJ that could change.
China Unicom’s CEO hinted Apple is preparing to ship an iPhone with WiFi in it, but he didn’t say when.
When Apple first entered the Chinese market, regulations barred mobile phones from having WiFi. Instead, the phones had to have a Chinese standard called WAPI.
After Apple redesigned its phone to include this standard, China changed the regulation so that phones with WAPI could also have WiFi. By the time the regulations had changed it was too late, Apple had manufactured phones without WiFi.
It looks like the next version will have WiFi, which ought to help sales, at least a little bit.