I think it's just an accident of timing and history.

The old 30-pin connector (inherited from the iPod) had various issues so I think Apple was eager to replace it. The lightning connector was their solution. It predates USB-C by a few years, so that wasn't an option at the time (I guess it might have been on Apple's radar by the time the lightning cable was introduced, but if so, they must have made the call not to wait.)

Since USB-C has made its way to some iPads, my guess is Apple is in the process of phasing out lightning connectors entirely.