All intuitive guessing on my part: I think slip based traction measurement used to be cruder in other vehicle designs. With electric motor you have ability to software control torque more accurately than mechanical. Edit: no, that might not be true. Anyway, perhaps the rest still applies: This allows you to effectively use wheel spin measurements that are not as gross and still measure relative traction. Think of turning radius fast vs slow in a regular car, or better tracking around curves at higher speeds by the trailers of tractor trailer combos: the wheels don't lose all traction, but do slide more. I'm sure this is electronically measurable.