Because China isn't sure of cultural ethnic mixing up?
The experiment in multiracial multiculturalism is just that. An experiment in progress. If inter racial strife in US and Europe is any signal, it's that this experiment hasn't supported the argument for more immigrants. China also has immigrants in HK. Much of the issues plaguing HK has been attributed to (covertly by the Chinese government and overtly by the people) on Vietnamese or Malay origin HKers along with Anglican culture. So that's another story that persuades China against considering immigration from a national security pov.
Then there's the Chinese themselves, who, like their east Asian cousins abhor "mixing up" (but not to the extend of Japanese). Definitely, unlike Germany, they aren't showing more affinity for immigration as time goes by.
I'm fairly certain that China won't institute immigration. It may institute a better visa regime for high technology jobs (just like US is doing these days). No citizenship I guess.