It's not that he's a bad player, it just that, there aren't many teams who are going to acquire Petry, let alone many of the top-end teams.
If we were to trade him tomorrow, the team acquiring him would need $6.25M in capspace to fit him in + the next few years, and they'd need to give up a pick(s) and/or a prospect(s) to get him.
I'm not sure there's going to be many legitimate teams lining up to get a Jeff Petry, those teams already have their own Jeff Petry equivalent and aren't looking for more high-priced talent, they're looking for players they can get on the cheap.
However, if the Habs were willing to absorb half of Petry's salary, now we're talking...