He calls Apple a rare example of a consumer-focused business with highswitchingcosts because customers are attached to its music, photo and other applications.
Though the answer to this question can vary from one region of the country to another, the most plausible answer is lack of competition on the supply side and highswitchingcosts on the demand side.
Make iCloud work seamlessly and easily so we store all of our important stuff (media, photos, videos) in iCloud and make switchingcosts very high. (iCloud now has 250M users.) Keep adding features to iMessage and promote iWorks to work with Microsoft Office so no one has a reason to switch to a Microsoft Surface.
It is reasonable to believe that the switchingcosts of an existing iPhone user are extremely high and that it is unlikely for one to switch unless there is an overwhelming advantage to do so.