- USBC is a generic port and it could be harder to implement authentication electronics (Which Lightning has) so Apple can keep charging for the MFi program.
- A portless phone has much better water and dust resistance
- Data transfer is the only drawback for the portless phone, but modern wireless tech like WiFi 7 are almost at thunderbolt speeds, so it could be that AirDrop gets upgraded with it
The point of USB is that it's universal standard, there's not going to be something proprietary like MFi. All the other Apple devices with USB-C already demonstrate this. MagSafe is its replacement.
Water resistance is not that important, I use the Lightning port pretty much every day, I never put my phone in water. I think it's safe to say that for most users the ratio of port usage to underwater usage is similar. As for dust I've never had an issue with it in the Lightning port. Cases are available if additional protection is needed in some situations.
Wi-Fi can be fast but it also needs to be reliable. Transferring files is not the only thing the port is used for, any wired accessories such as headphones, microphones, some battery packs/banks, card readers, CarPlay and others use it as well.