Lightning was first, there is a vibrant ecosystem in place around Made For iPhone accessories that use Lightning that a lot of existing iPhone users already own, the Lightning port and connector are smaller than USB-C which is probably useful in a few places (I saw someone suggest somewhere that the Apple Pencil wouldn't even be able to fit a USB-C connector), someone else also said that Lightning was designed to support the weight of the iPhone in a dock configuration but USB-C probably wasn't, etc. And there's not really any benefit to switching to USB-C besides being able to re-use Android chargers, which isn't something that Apple or most of their customers particularly cares about.