Once upon a time, there was a hockey player named Marc Savard. Savard was a good player and deadly on the powerplay. It was theorized, give Krejci or Bergeron the same power play time, and they will score the same points on the powerplay. Unfortunately, the hero of our story was injured, and Bergeron and Krejci did get that time. What transpired was the worst powerplay of all time. Also a cup, but anyway...
However, out of the land of Michigan was a hobbit by the name of Krug. What Krug lacked in size or strength he made up with vision. Once he had a new general, Krug started to lead one of the best power plays in the NHL, and it became a staple of the team's success.
Moral of the story, being good on the powerplay is a skill not every player has. Krug has a special talent offensively and I am fairly certain the Bruins don't have a defenceman who can replicate that success.