Short answer:

There was no USB-C back in 2012 when Apple shipped Lightning on iPhone 5. It didn’t exist. The spec wasn’t even finalized until August of 2014.

But there’s more to this article. I especially appreciate the overlay showing the relative footprints of USB-A, USB-C, and Lightning.

Will the iPhone ever move to USB-C?

USB-C would require another port change for customers. Many people weren’t very happy with the last one, and Lightning was 10 years after Dock. It’s only been 5 years since Lightning. And in that time, with hundreds of millions of devices on the market, Lightning has become ubiquitous enough that everyone has it, typically in abundance.

Interesting that the Mac has made the first move, going all-in on USB-C. I wonder if there’s a prototype USB-C iPhone floating around an Apple campus somewhere.