Because they promised thr connector wouldn't change again quickly when they introduced lighting.

And consequently broke that promise by not using Lightning on Mac?

Mac never had a 30-pin adapter, so lightning makes no sense. Mac also had Thunderbolt 2 with the miniDP port, and switched to Thunderbolt 3 with a USB-C port.

Prior launches had more important priorities to include so it was easy for teams to push this feature back knowing that the only mandatory launch date for it to hit was the iPhone 15 Pro launch.

Having the clarity over what's important allows teams to perform rather than endlessly debate over tradeoffs.

I question the complexity of the switch from lightning to USB-C, do you have any resources or knowledge to help clarify that part for me?

It still feels like a shaky premise to me because had USB-C not been mandated, they'd never have prioritized it. I think they waited until the last minute simply because they could.

The iPhone 15 doesn't have 3.0 USB-C because the silicon was only introduced in the A17. Hardware revs are set in stone many moons before so being able to push USB3 support a generation later allows engineers to focus on other tasks.

Again, all speculation but the point is how a defined end point allowed teams to receive clarity over tradeoffs and move quickly. It's very typical of how Apple works (which you can see in Vision 1.0 where certain things were wildly overspecified and other were wildly under for that reason). The exact details may be wrong but the logic of how they got to this place feels right.

Because they make money off every Lightning accessory sold.