Early on it was due to price, and the main contribution from China was in low cost final assembly work. For the first half dozen or so iPhones about a third to half of the value of the device, including for several models the CPU (Samsung plant in Texas) and Gorilla Glass (Corning in NY), was in components actually manufactured in the USA. The cost of final assembly in China was only a few dollars per device. These variables have changed though as Chinese manufacturing technology and skills have steadily climbed up the value scale.