Because... You'd naturally play your best defensive forwards on the PK, or as the Selke trophy puts it: the forwards who demonstrates the most skill in the defensive component of the game.
In the 2010's no player has gotten the Selke with less than 100 minutes of PK time(Well, Toews did, but that doesn't count for obvious reasons, and I think the minimum PK time number is even higher at around 114 minutes). Crosby is nowhere close to that. He has 42:40 on the PK this season, which obviously asks the questions: why isn't he getting the time on the PK?