Probably the only reason. If they hadn't demanded it, it wouldn't have been conceived and no one else would have asked for it.
Agreed. And in the beginning the JSF was led by a Marine general with the Marine requirements the priority. It was supposed to be lightweight, simple, and low cost. All that changed when USAF took over the project. They had a very different vision of the aircraft than USMC. Eventually the Marines were able to reassert themselves and that's why the B later got priority development. But the shifting requirements and priorities took their toll on the schedule and on cost.