"You must use the standard, unless doing so would have a substantial impact on your product's capabilities." (I wrote this in 30 seconds, replace with more legally sound version.)
I feel like Apple would have a hard time arguing the lightening is technically more capable than USB-C. But if they want to create something that is a clear improvement, they can go for it... until a standardized version catches up.
I think another good argument for phones is that it's surely much easier to clean lint out of a lightning port than a USB C port.
Edit: But probably, I'd guess, right now the main reason Apple has stuck to lightning to whatever extent it is is because it's more convenient for its customers. USB C is already the standard, and Apple would benefit from switching to USB C across its product line if its customers would like that more.
Regarding this specifically, I think it's because Apple makes money from licensing Lightning cables and lightning accessories.