More students have cars now than in the old days which gives them more options on where to live. I think maybe 10% of the students had cars in the late 1970's.
Don't a lot of football players move off campus by junior year?
Many schools keep seniors on campus by having really nice "senior dorms" with kitchens and bathrooms in the unit - essentially apartments owned by the university. That cuts against the ND model of staying in the same dorm 4 years.
This also might be a money thing for ND. They have built all of these new dorms and if there is a possibility they might have empty dorm rooms, that will hurt revenue. I think we have all learned that most things at ND are about maximizing revenue.