I question this premise. The 1970s, before the playoffs expanded, were dominated by six teams, viz.:
1972
NFC: Cowboys, Redskins, Packers, Niners
AFC: Steelers, Raiders, Dolphins, Browns
1973
NFC: Cowboys, Redskins, Vikings, Rams
AFC: Steelers, Raiders, Dolphins, Bengals
1974
NFC: Cardinals, Vikings, Rams, Redskins
AFC: Steelers, Raiders, Dolphins, Bills
1975
NFC: Cardinals, Vikings, Rams, Cowboys
AFC: Steelers, Raiders, Colts, Bengals
1976
NFC: Cowboys, Vikings, Rams, Redskins
AFC: Steelers, Raiders, Colts, Patriots
1977
NFC: Cowboys, Vikings, Rams, Bears
AFC: Steelers, Raiders, Colts, Broncos