Likely because he is the most well rounded partner Petry's had this season. Smid, Schultz, and Ference bring zero ability to move the puck, and a guy like Belov isn't really fashioned to be on a shutdown pairing.
Put Petty with a true #1 and it'll be a functional top pairing, or have him with a guy like Smid, and it can be a good shutdown pairing. Ask him to be a #1 and carry a #4 (or worse) against top competition and he starts to mentally break down.
This team really needs a leader to its defence, this isn't like years past were I hate players on the defence, and how they weren't NHL calibre. There's no Foster or Studwick this year, its just that now everyone's being asked to play over their heads, and beyond their ability.
I'd love to see this team land Boyle in the off season. He bridges the gap between now and the defence of Nurse, Klefbom, Marincin, (possibly) Ekblad, and Schultz nicely, and like Ference, he can step into that mentor roll.