Apple's timing with the lighting port wasn't great, with it arriving just before USB-C, and I imagine they're very reluctant to change the port on their phones because people already have lightning cables everywhere.
Personally I would love to see USB-C on my iPhone, but I look at my parents, and having the port change for them would be a pain in the arse. All their cables are lightning cables, they don't have any USB-C devices, and ofcourse, all of their cables are the ones that came in the box. I suspect a significant portion of Apple's customers fall into that category.