Apple is lagging behind in terms of display technology. Apple now depends on Samsung and others for these news flexible displays. Apple needs to develop its own technology in display, flash memory, batteries etc, rather than repackaging what's available on the market.
China is developing so rapidly, that the workers have many other vocational choices. Chinese wages are also rising rapidly. Demand for Apple products in China is staggering. As a result Foxconn needs to have extra capacity elsewhere.