There was mental aspect to the Steelers Cowboys in the 1970s that lasted into the early 1980s. The Steelers won 5 in a row and it seemed like Dallas simply couldn't beat them. They could beat everyone else, even teams that could occasionally beat Pittsburgh like Denver, but they were whipped dogs with Pittsburgh.
Not only the two SB losses, but a loss during 1977 where Dallas looked like they might go undefeated, and they did beat Denver in the SB.
In 1979 they lost twice to Pittsburgh, SBXIII in January, and a regular season loss in October.
There was also a loss in the 1982 strike season when the Cowboys were a much better team on paper.