I think you also have to consider the strength of each league's player's unions as well as the fact that the owners are really the ones the commissioner works for. Whether or not Selig genuinely cares about getting the steroids out of the game he definitely has the strongest union fighting testing and shit every step of the way. And a salary cap/floor system that would go a long way in fixing some of the fucking laughable disparities in baseball like the Yankees and the Astros.
Bettman and the NHL owners, on the other hand, have bent the players over in at least the last 2 CBAs..even though it took work stoppages to utterly break them both times. Utterly broken is the key phrase there. As far as the owners are concerned, Bettman is probably the golden boy. Ask Jeremy Jacobs how mad he will be with successfully raiding the players share of the revenue AND potentially winning the Stanley Cup this year.
I guess my point is that it's harder in some sports to really get your agenda through the players than others, and it's hard to hold that against the commissioner.