Well, I can answer this at least partially. The lighting connector:
- is a smaller plug and allows for smaller devices
- is more easily waterproofed
- as something Apple certifies, can have quality standards
- is already deployed; no complaining about people having to upgrade all their cables or confusion when people shop for iPhone accessories
Two other issues:
You can't have an official USB-C female to USB-A male adapter, e.g. connect a new USB-C cable into a USB-A device. It is not allowed by the USB-IF for device safety reasons - pre USB-C, there weren't protections against devices being plugged in incorrectly, which was why they had different-shaped plugs. With a USB-C adapter, you now can plug devices together in ways that were not allowed, and which the devices are not designed to handle.
This is IMHO why a lot of companies have been slow to adopt USB-C cabling in the packaging - the end-user can always use an adapter (heck they probably have one) if they want to connect to their USB-C computer, but the vast majority of users with older computers would need a different cable.
Second, which is more of a conceptual issue - USB-C is far closer to the 30-pin connector in terms of design rather than lightning. Do you remember the tables for things like the video adapters, e.g. which models had the hardware support to make which video adapters work?
Apple solved this in lightning but having a common serial protocol from the device out, e.g. H.264 video data. The adapter was now a piece of complex hardware itself, responsible for taking that data and converting it to VGA/composite video/HDMI - whatever it supported.
Say they later want to support 4k? They can make both the iOS device and the new adapter backward compatible, so the story stays pretty simple - hook a 4k HDMI adapter up to a 4k-capable iOS device and you're golden. If either doesn't support 4k, it will fall back to 1080p (but still work).
USB-C has at least FOUR different methods used for HDMI out; some of which require extra hardware, and the cheapest of which requires a proprietary software driver. There are no markings to say which adapter(s) work with which hardware device(s). To give an acceptable user experience, Apple would have to either:
- Tell people to only use the Apple HDMI adapter, any other version (may not) be supported
- Make the iOS device substantially more complicated, and hope nobody launches a fifth method for HDMI out.