The reason is that like with the iPhone 14, in the non-Pro models they put the SoC from the previous year's Pro model, and that one was only designed for Lightning so only USB 2.0. So the non-Pro will get USB 3 once the USB 3-supporting SoC trickles down from the Pro.
Apple is very good at price discrimination. I hey know if they can build a slightly cheaper phone by reusing the SoC from the older lightning version, and 99% of iPhone users won’t care (for whatever reason) they then know that the 1% that does care will spend a little bit more on the Pro model. And they do that with few different features, which ends up with the Pro models selling a significant number of units.