I think what it comes down to is 2 things
1) Tomlin is a victim of his own success. He's had tons of regular season success and people expect the same type of success in the playoffs. Which is understandable. However, they try to downplay his success and accomplishments because they don't dominate their opponents. No NFL team dominates their opponents, however, the Steelers have the best point differential in the NFL so they are currently the closest thing to a "dominant" team in the league.
2) People don't want to give Tomlin credit when things go well, but are quick to bash him when things are going bad. He's not a passenger or an intern, he's the head coach. In fact, if he keeps going at this rate, he may end up in the Hall of Fame.