Growing up and living in my area Notre Dame had a presence, Pitt had none. and at that time most of the games were played. In the eastern part of the state Pitt is a non issue, people just don't care.Notre Dame is the second most popular team in a lot of parts of the country but it's hard to call them a PSU rival when the two teams have only played 19 times going back to World War I.
That is your 20th most common opponent. 20th!