Comment by jdietrich

There's a much more mundane explanation - waterproofing. Lightning and USB-C connectors can both be made intrinsically waterproof up to IPx7, while the 3.5mm jack can't. Waterproofing is a key point of differentiation for recent flagship phones. An iPhone 7 will survive a dip in a toilet bowl or a pint of beer, but an iPhone 6 probably won't.