Apple could easily make 2 types of iPhones and let users choose - USB-C or Lightning.
Besides the logistical issues in producing, selling and marketing two different connector models, it also means third party accessories need to support these connectors. Want that new FLIR camera for your phone? Please choose between Model A or Model B.
One connector may also end up being the primary, where third parties only create versions of their hardware for that one due to the user share, effectively eliminating the other one and pissing of your customers.
Besides most of accessories are disappearing with Bluetooth, Wifi, cloud, etc.