Not saying that these are valid reasons however two that spring to mind are thickness (USB-C is fairly significantly thicker) and elegance (one could argue that Lightning is a more beautiful and slightly more pleasant to use connector).
In addition, Apple controls Lightning completely so when you buy an accessory that uses Lightning, as long as it isn't counterfeit, it has been tested by Apple and is guaranteed to work. They can't guarantee that with USB-C.
That said, it's an awkward situation. Overall I'd like USB-C on my phone however if they were going to do it, surely it would have been with the iPhone 7. It wouldn't be great if they announced it with the 7S after people had gone out and bought Lightning headphones.
In addition, Apple controls Lightning completely so when you buy an accessory that uses Lightning, as long as it isn't counterfeit, it has been tested by Apple and is guaranteed to work. They can't guarantee that with USB-C.
That said, it's an awkward situation. Overall I'd like USB-C on my phone however if they were going to do it, surely it would have been with the iPhone 7. It wouldn't be great if they announced it with the 7S after people had gone out and bought Lightning headphones.