ND is only willing to lock in so many games per year as they want to play a national schedule. Outside of the late 80s, the Michigan rivalry hasn’t been as big as others. They've actually only played 38 games against each other.
USC is untouchable, that is always the big game. Navy shares the same tradition, nearly 100 years now.
ND schedules other private schools that allow them to connect to alumni across the country, so Stanford and BC have grown into key rivals.
As for regional rivals, ND has a much longer history with Perdue (on hold), MSU, Pitt and Northwestern than it does with UM.
ND has also has shorter series against a ton of major programs, also with the goal of playing teams from across the country (UGA and OU recently, but just about every big team at one point or another). Two of those that go way back are GT and UNC, which have been rekindled lately with ND being part-time ACC members.