This is simply the obvious, non-stupid thing to do, like Apple switching to USB-C.
From a profit maximization standpoint you want identical phones that can't easily be sold across market borders. Usually that's done in software (e.g. region locked video games), but that can usually be cracked, so a single design difference is the best solution. A great example is Nintendo's cartridge slot shapes that were different in each region despite the hardware being the same.
Apple had lightning everywhere; they were forced to change to USB-C. Therefore they changed to USB-C everywhere. The market protection design change remains the sim slot.
OK, so why not ship a SIM card slot in the US? eSIMs work perfectly well in Europe, the US and most of the rest of the world, "region locking" is clearly not the answer.
Apple did not have "lightning everywhere" either - Macs and iPads got it much earlier.