Because most PC peripherals (mice, keyboards, microphones, controllers, USB headphones, detachable hard drives) are still USB-A on the other end of their cable. Yes this is changing, but in this case I appreciate them acknowledging the reality on the ground and not creating a situation where there are many dongles afoot.
Most peripherals these days have a fully detachable cable so it can be whatever you want. Usually they include some garbage 10 cent C to A cable in the box that you throw in the bin and replace with a good quality C to C cable.
I could understand having both C and A on the machine, but having the one C port on the back seems like a mistake since that's the port I'd think you'd most want quick access to to plug in flash drives and such if you wanted to copy files from a phone or laptop. While the back ports are where you'd stick the fixed receivers.