One reason I can think of: a 99 cent one off of alibaba might deliver a 100 volt jolt directly to a device that is electrically connected to your brain, killing you instantly.
I'm all for universal standards, but this really does seem like a case where the connection should be a clean sheet (maybe even proprietary) design where safety parameters can be tightly controlled. My guess is that they're using USB during the development phase for ease of development (they probably just haven't spent the time on making a different connector yet). I would be very surprised to see this actually go to market with a USB-C port. Given the notorious issues with USB-C cables, I would be surprised if the FDA even approved a device with a user-accessible USB-C port.
