With a salary of $6.25 million per season through 2024-2025, it's understandable that he was hard to move as La Presse reporter Mathias Brunet mentioned. He mentions that if Petry had been an uncompensated free agent at the end of the season, as Chiarot and Kulak were, Kent Hughes would have been able to potentially get a first or second round pick at a minimum.
On the other hand, we learn that due to his hefty contract, Petry had a negative value on the market instead... Ouch! Kent Hughes would have had to pay to trade him, which is definitely not ideal.
Probably a Stanley Cup contender would have been willing to pay a good price to acquire a defenseman of Petry's caliber, even if he's having a sub-par season, if his contract expired at the end of the year. That says a lot about the impact and importance of the salary cap.