Steelers got lucky in the two games they won against Dallas in the Super Bowls. Especially the second one, when Jackie Smith dropped a wide-open TD pass, there was a "phantom" pass interference call which WOULD NOT BE CALLED in today's NFL (it's now called "incidental contact"), a fumble on a kickoff by a lineman with a cast on his hand, and the umpire getting in the way of a tackler to spring Franco Harris for a TD run where he should have been tackled. Steelers had a GREAT TEAM, and EARNED the title of "Team of the 1970's" with their 4 SB wins. But Dallas got jobbed in that second Super Bowl for sure.
And the "America's Team" thing? The other teams just REFUSE to accept that YES, Dallas WAS THE MOST POPULAR TEAM based on sales of gear, TV ratings, etc. Did that mean ANYTHING ELSE besides the fact that they were the MOST POPULAR TEAM? NO. They were merely the most popular team, and that is a fact that no resentment about the "America's Team" moniker can change. It's HISTORICAL FACT so quit whining about it, quit punching them in the face about it. It was nothing THEY did - it just happened.