It's precisely because MLB self polices so well that this is extremely unusual and we're even talking about it.
No, baseball has less brawls because, at heart, it's not a violent sport. The NBA is a lesser violent sport than the NHL but still involves much more physical contact than baseball. You tend to have more violent altercations happen in sports where violence/physicality is engrained in the game. In baseball, there need be no violence at all really. As demonstrated by many levels of baseball where it is played without any issues.
Look, you're welcome to like self-policing for entertainment. But it is demonstrably ineffective and self-fulfilling.
Meanwhile with some rule changes the NHL has all but made the enforcer (read: self-policer) a thing of the past. And the game is better for it.
Edited by TheLeviathan, 19 May 2016 - 07:44 AM.