When the league championship is the big one in a long old season, and every night you have to come out with that mentality, then it is just human nature to take your foot off the gas when the championship is done.
Add to that the significant injuries we have, the remaining players not wanting to risk injury for themselves for one cup there's left, and also wanting to restore some energy at the end of a season, then this performance and probably the remaining league ones will obviously not be the same as the Steelers game.