The answer? Educated people and wages.
See Gruber’s article here. I think that discussion becomes very interesting as wages rise in China. As Gruber points out the jobs these folks are doing might be defined as ‘menial’ but the jobs take skill. So the Chinese market offers the both skilled labor and until recently, inexpensive labor.
As China’s economy changes and wages become higher low cost assembly will (and is moving) move to other countries. Do other countries offer the same combination that China does or is the ‘savings’ that a company takes partially eaten by training of employees?