Because every designer has a choice - build onboard A connectors and supply seperate A-C adaptors or build C-onboard and supply seperate C-A adapters. The ince

Because every designer has a choice - build onboard A connectors and supply seperate A-C adaptors or build C-onboard and supply seperate C-A adapters. The incentives are to make the motherboard at a price point and A connectors are going to be 1cent or less cheaper. This is probably true for the cable adaptors too. But if I had to put money on one thing it's power - you can run a small laptop off USB-C which is where the whole thing shoukd be moving to - USB-C for everything power and data, but making every connector hook up to the 5.5V on the motherboard is awkward and marking just one is confusing. It's not a great argument but my take is USB-C is only great when every connection is USB-C and if it's not great then why bother