I agree with your assessment: it's easier to find subjects with peripheral nerve issues. It gets progressively harder to find subjects as you move up the chain, as the risks compound quickly: peripheral > central > brain.
As you can imagine, neural link is targeting some of the most invasive -- hence animal subjects.
You don’t have to go far to find candidates, walk into any ER today and you’ll find trauma victims with irreversible nerve damage from a common car/motorcycle accident.