If someone has proven they can do it successfully multiple times in different scenarios, they can probably do it again. There are high stakes when a coach is hired. The GM is putting their own job at stake. The owner is putting the success of their team at stake. So they likely want to go with a known quantity. It's also a specialized position with a high barrier for entry. While there are lots of coaches in the world, it's not like the coach of you sons Junior B team has shown anything that would make someone at the head of a close to billion dollar corporation comfortable with them handling the coaching (they forget to bring the oranges half the time...or are oranges only in socccer?).

Hockey is such a specialized sport with coaching, management, even something like being backup goalie or fringe NHL goalie, once someone gets their foot in the door and proves they can do it they are likely to get more than one chance somewhere. It takes a lot for someone to really screw that up (google Pierre McGuire)