The explanation has to start with the understanding that the best time to get a recruit on campus is for a game day experience (assuming you are good and the stadium is full - which was true back then). Further, the best place for any school to recruit is within a 3 hour drive of campus. Schools in talent rich areas (TX, FL, LA, CA, GA, AL, OH, MI, PA) were not affected as much by this rule as many of their recruits could play Friday night, get up Saturday morning and get to the campus by car in a couple of hours. IN, particularly back then, did not have much high D1 talent. Joe was "making his living" in TX. The Purdue airport gave us an advantage over other schools in the Midwest that were recruiting TX kids. Because we had the airport on campus, and the aircraft, we would have a plane pick up a half dozen kids in TX that all lived within an hour of each other (at a central location) and get them up here after their games on Friday, and in plenty of time to experience a full game day experience. It gave us an advantage over other schools in the B1G that were also recruiting the same kids from TX; because we could get them on campus for a game day experience, and the other schools could not.
Once the rule changed, the advantage we had for those kids - versus other schools farther than a car drive that were also recruiting the same kids (that did not have an airport and planes on campus and available) - was lost.
That was Joe's point.