Ed Bouchette explains why Mike Tomlin challenged and didn't challenge those respective plays in his chat session:
steelerfaninNM: Ed, what do the coaches see differently in the booth than we do at home. MT challenges an obvious down by contact play on a punt return and then fails to challenge an obvious Mendy touchdown run on that fateful 1st and goal series in which Mendy was tackled on top of the defender and roll over with the ball clearly crossing the goalline, what gives?
Ed Bouchette: I figured this is what happened and after further checking today, I found out it is true. The television in the visiting coaches booth in New Orleans was tuned to the scoreboard view of the game, which is controlled by the home team. The home team will never show a replay if a successful challenge would hurt the home team. Thus, they did not show the replay on the video screen at the game and so the Steelers coaches never saw what we all saw -- the NBC network telecast that clearly showed Rashard Mendenhall had scored. Tomlin had no idea because he did not see it clearly and no one from the Steelers ever saw a replay. He should have called me in the press box because I saw it.
Ed Bouchette Chat Transcript 11.02.2010