Because asset and talant management is a HUGE part of the role. There is no way, zero way that they knew what they had Marchand and Lucic. He He also traded for Chara, Peverly, Horton, Ference, Boychuk, Kelly and Seidenberg, who were all vitally important to that run.
Hate him all you want for the Oilers, but his fingerprints were all over that 2011 team.
There's also no urgent area on that Boston team that needed him to really work hard.
He just had to find role players, he got gifted Bergeron, Krejci, Kessel/Seguin, Marchand, Lucic, Thomas, Rask from day 1 and then used inside knowledge to get Chara from day 1. But that's not a repeatable management strategy.
In Edmonton he had a D-corps that had some big immediate issues, and he's been exposed as someone who doesn't know what he's doing. He panicked and traded for Reinhart and then got destoyed in a Hall trade and thought Lucic upfront would be a pivotal piece of his core, and in doing all that basically destroyed his forward depth too. Now he's stuck with a team where the forwards and D are both underwhelming.
It's not that hard to GM a team when you have great depth at center, good options at wing, a bonafied no.1 Norris contending D in his prime, and 2 good goalies from the start. There's no pressing huge need anywhere on that Bruins roster, he just had to add in some depth pieces.
In Edmonton where he was actually required to do some hard work, he's been exposed as a fraud.