Most likely because of the Americans with Disabilities Act (ADA) and the international equivalent to ADA, which requires "equivalent access" in transportation facilities for all. In some cases, where airlines still use stairs or built-in airstairs at for example, DCA, the airline will still need to provide access to the mobility challenged via some equivalent means as they do with able-bodied persons. Airlines would probably provide that if the plane is using a built-in airstair from the wayside, as needed, that is, if they know a passenger on the flight is mobility challenged.