An attribute is not a species' differentia if removing it leaves the specified nature intact. Black hair or currently walking does not constitute humanity.
In the classical definition, rational identifies an intrinsic constituent of what the human species is. Without that determination, its account within the genus remains incomplete.
This does not claim that each human is constantly reasoning. A differentia concerns what a species is, not an activity observed at a given moment. This matters when comparing it with properties and accidents.