Because they are two of the oldest franchises in the NFL with ownership that has been in the same families for decades, they has always been a tie between the teams. They actually merged for a season (in 1944 because of the war) and of course matched up in the Cardinals' lone Super Bowl appearance for Super Bowl XLIII.
That's one of the reasons the passing of Steelers owner Dan Rooney Thursday was felt around the Cardinals -- not to mention that Rooney was one of the most beloved owners across the league.
"This is an incredibly sad day and profound loss for all of us who had the pleasure and privilege of knowing Dan Rooney," Cardinals president Michael Bidwill said in a statement. "Mr. Rooney's love for the Steelers, their players and their fans was apparent to everyone, as was the tremendous pride he took when he was appointed U.S. Ambassador to his beloved Ireland. But all of that was exceeded by the love and deep devotion he had for Patricia and their wonderful family. Our thoughts are certainly with the Rooneys, their entire organization, and all of Steeler Nation upon the passing of this remarkable man."
Cardinals coach Bruce Arians tweeted out his sentiments, after coaching with the Steelers for a number of years before coming to Arizona.