One main reason is simplicity. the same port on all devices sounds like Jony Ive's wet dream. And it would be easy to use, I wouldn't have to worry about what cable I have or what charger. Also if the apple pencil becomes USB-c it can charge from the Macbooks. and if someone owns android products, they can charge those as well.
How long will apple stick with lightning for the iPad?
I'm not sure about Jony Ive's dreams. Lightning uses a 16-pin connector/cable, USB-C is 24 pins. For someone who likes "cleaner, lighter," I'm not sure "more pins" is what he dreams of.
I predict that you will continue to be concerned about what chargers you carry. You'll have small, low-power chargers for mobile devices, and larger, high-power ("PD") chargers intended for laptops or the occasional USB-C Bridge or disk drive array. Do you really want to carry around several laptop-sized chargers, and carry only PD-certified cables (which are bulkier and more expensive), if only one item in your kit requires a PD-strength charger and cable?
You're also equating physical compatibility with
functionality. The USB-C standard does not require a manufacturer to provide all potential capabilities when it uses USB-C. For example, a device with USB-C may provide analog audio output for headphones, or it may provide digital-only. Meanwhile, a person with a pair of analog headphones with a USB-C plug on the end isn't going to get very far plugging the headphones into a standard, digital-only USB-C port.
Similarly, a port may be capable of driving Thunderbolt 3, or not. It's all allowed by the "standard." There's a growing list of "Alternate Mode" specifications.
It's not unlike A/V equipment that use RCA jacks for nearly all audio and video. Plugging an audio device into the video jack isn't going to get you very far, and taking a composite video output from one device is pointless if the device to which you're connecting only has separate R, G, and B inputs. Sure, life is simpler when you can use the same cable for many purposes, but when all the cables look the same, it also complicates the interconnect process. That's why the RCA cables that come with A/V gear are color-coded.
To me, USB-C delivers the
illusion of universal compatibility. Who cares if a single USB-C to USB-C cable can be used for any purpose, if the iPad doesn't support the purpose you have in mind?
It seems a fair portion of the "I want USB-C" sentiment in this thread carries an assumption that a USB-C port will bring every traditional computer i/o capability to iOS - external displays, external storage, ethernet, hard-wired printers, etc. Not likely at all. Apple's i/o and storage scheme for iOS is quite intentionally limited, there's still no reason for them to go back on their original design philosophy - they could have done all of that, years ago, without USB-C.
This is just another, "I want iPad to be a Mac" thread, substituting "USB-C" for "MacOS."