Budget reasons would be my guess. The USB-C port here is 10 gbit while the USB-A ports on the machine are USB 2.0(front) and 5gbit(rear) speeds.

While they could have and imo should have just put usb 2.0 C ports on it, users have an expectation that USB-C ports are fully functional high speed ports. On most machines you can plug something in to any USB-C port on a device and they all work the same.

Because most peripherals that are likely to be used with it are still USB-A. Things like mice and keyboards and gamepads and headphone and whatnot.