It’s annoying but pretty clearly about differentiating the Pro model, at least at first. This has been Apple’s strategy at least since the XS/XR: the high end model gets a better camera and higher performance SoC to entice consumers who want the best as well as the actual professional users. And recently they’ve apparently moved to having the base iPhone lag a year on the chip. USB3 requires a specific chip so the Pro gets it first. The question is, does the base iPhone get USB3 with the A17 series SoC next year, or does it join the camera as something to differentiate the Pro series? It would make sense to keep it in the pro model since fast USB transfers is something few consumers care about but pros care a lot about.