There's so much stupid involved in the Steelers crying about losing the 2004 AFCCG, it's hard to know where to start...
Pittsburgh had a terrific regular season, going 15-1 and beating the Patriots in Pittsburgh. There were several key personnel differences between the regular season and postseason match-up, though. Corey Dillon missed the first meeting due to injury. During the game, Matt Light and Ty Law went down with injuries. Take away a team's #1 RB, left tackle and #1 corner, and they are going to struggle. In the playoffs, Light and Dillon were back and the defense had time to adjust being without Law.
The week before the Patriots played Pittsburgh, the Steelers barely defeated the Jets. The Jets had played in Oakland the week before, then had to go to Pittsburgh. Like the Patriots, the Steelers had beaten the Jets handily in the regular season. In the divisional game, though, the Steelers escaped with a win in OT. They were extremely lucky to get to OT and win as the Jets missed 2 FGs that would have won the game late in the 4th quarter. I have never heard anyone from the Steelers accuse the Jets of stealing their signals as a reason they struggled.
The Steelers players and fans seem dumbfounded that the 2004 team didn't win the Super Bowl. They look at the Patriots and say "How could they do it? Something must have been going on." They couldn't imagine the Patriots beating their team. On the other hand, they had no problem accepting that their team would start the season 1-1 and then win 17 straight on their way to the SB with a rookie QB leading the way. As good as Roethlisberger played in his first season and beyond, he didn't come out of Texas, USC, OSU, or any of the other powerhouse programs of the mid-2000s. He played at a MAC school and faced less than 10 schools from the Power 5 conferences in his 3 years at Miami (OH.) To expect him to win every game he started in his first season in the NFL, including 3 playoff games, isn't very realistic. Put another way, if Cleveland, Baltimore, or Cincinnati drafted a QB from Akron that QB guided the team to 13 straight wins in the regular season, Steelers fans would find that very suspicious.