There is some ever-so-slight chance that losing Lightning for USB-C is part of some other, larger strategy involving broader peripheral use on iPhones and especially iPads.
When the iPad Pro came out there seemed to be a lot of marketing trying to sell it as some kind of Laptop Lite, which is tough sell with a hermetically sealed system that doesn't support (AFAIK) even bluetooth mice other than a couple of specialty models and even then on an app-by-app basis, let alone various USB thumb drives or other basic USB devices.
Maybe they see some synergy between peripheral compatibility, parts sourcing, and use-case expansion as somehow key to expanding market share. By switching to USB-C they can further cut overhead costs (USB-C sockets in like quantities have to be cheaper than Lightning ones), roll out some kind of additional peripheral compatibility and maybe grow their iPad sales, or at least stave off losses to Wintel tablets.
The switch to USB-C would make less sense in the context of phones vs. tablets, but would it be expected (or even demanded) by consumers that phones and tablets would share the same connector, as well as unify the entire product lineup from phone to desktop.