Pavelski comes off as being a great leader, IMO, because he is one of the guys who has really had to work on his game significantly to even make the NHL. Boyle and Couture are somewhat similar. The trio come off a little bit as having chips on their shoulders...
I think we see that extra work ethic, desire, spark, etc., and call it leadership. I don't know if it is fair. Maybe you can say that those players are harder workers, but leadership is a different thing.
Another reason, is that one of the visual ways to demonstrate leadership, is leading by example. Pavelski/Couture/Boyle exemplify this...but there are other ways to be a leader.
Plus, there is leadership vs. mentorship. For example, I would say that there are decent examples of both Thornton and Marleau taking younger players under their wing...how much they really helped is unknown, but there appears to be some level of mentoring from the two players. On the other hand, a player like Boyle doesn't have a great history of helping develop other defensemen, while players like Blake are universally praised for that quality.
Lastly, I will say what frustrates me about both Thornton and Marleau is their tendency to quickly head to excuses. I know some of it is hockey/team-speak...but at the end of the day, considering the talent level of the team "we tried hard and got unlucky" is not something I want the players to be saying. That is something that the fans rationalize. Players should give themselves no excuses. Couture and Boyle in particular don't seem to have those kind of excuses.