Because no one else is... If you're not going to hold guys like Zuccarello, Johansson, Gaudreau, etc accountable all season because you think they need to maintain their minutes and linemates to turn it around, don't suddenly change that philosophy with Rossi.
I don't disagree, plenty of people were calling for certain guys to be demoted all season and they weren't.
No one is mad about Rossi being held accountable, they're mad at the double standard that seems to exist, and that Rossi is the primary "victim" for lack of a better word.
Although I will concede that Hartman is also held accountable, it's not just Rossi.
But Zuccarello has never been held accountable in his six seasons here despite being a downright liability at times. Gaudreau maintains his role and ice time when he's on a 5 points in 27 game run. Johansson couldn't be moved off the second line with the jaws of life for whatever reason. And in the playoffs that exact thing was on full display for anyone who watched. Rossi got demoted, fair enough, he wasn't good to start the series. By the end of the series he was much better, and most of the bottom 9 was horrible, but there were minimal changes in role.
If everyone was held accountable the way Rossi is, no one would be upset when Rossi is held accountable.