Technically, one of the reasons why Apple won't move to USB-C for iOS has to do with the implementation that they achieved when the connector changed.
If they were to revert to USB-C, they would loose the ability to move the connector's actual conversion hardware out of the device and into the cord. Using USB-C, the devices on either end can be a host or peripherals but using Lightning, the cable itself serve as a host. We see it in the HDMI conversion cables - an iOS device shoots airplay compressed digital video down to the cable and the cable itself does the conversion and handles all of the outputting to HDMI. This places the complexity of transcription and relay outside of the iOS device itself, making them cheaper, and allows an unlimited connection or dongle potential unlike a USB implementation which requires the device itself to support the specific type of transfer (easier for computers to support HDMI-Alt mode than mobile devices, for example). There are other technical benefits of the ecosystem that Apple takes advantage of, like flashing firmware to cords and not depending on USB's form-factor (they can seamlessly update Lightning to use future USB implementations for faster transfer, for example). It also seems like Apple has implemented the ability for hardware to make digital handshakes using Lightning, rather than just some underlying system implementation on both sides (check Activity Log, its super cool!)
One could argue that Apple could achieve the same process using USB-C, but the separate standard makes this cord selection nearly seamless since they all implement the proper specs by conforming to MFi program or cloning those that do. Not to mention, with iPad on the verge of loosing weight and PC-like abilities, they didn't wait until USB-C was finalized to change it.