Each sport has its share of odd rules. Like in the NFL, why isn't spiking the ball considered intentional grounding? In the NBA, why are teams magically granted the ball at half-court after calling a late time-out in the backcourt? How come a player is down immediately when he hits the ground in college football, but must be touched down by an opponent in the NFL?
With MLB playoffs right around the corner, we will once again ultimately pit the top teams from the two leagues and decide a new champion. When the American and National Leagues battle for the World Series each fall, it reminds us how the two leagues are more independent from each other than are conferences within the other major pro sports leagues.
In the NBA, the conferences have nothing to do with the fusion of the old American Basketball Association and the NBA. The NHL is not the product of a merger, but rather many expansions to compete with rival leagues. In both cases, the eastern and western conferences in these leagues were configured based on geography as a means to cut down on travel by minimizing the distances between organizations. Therefore, teams are grouped based on region, rather than historic affiliation with past leagues.
The NFL's two conferences were derived directly from the merger of two rival professional leagues. The movement of some teams to even out the conferences, the relocation of teams and expansion of the NFL over the years -- plus the fact that interleague play has always existed in the NFL -- however, has saturated much of the rivalry between the conferences.
The independence of the two leagues within the MLB came out of the combination of the two competing leagues in 1903. One might assume that after such a long time, there would be fewer recognizable remnants of the rivalry that existed between the AL and NL. However, thanks to odd rules like stats not carrying over and the DH, the leagues remain rather separate.
When someone is traded mid-season from one league to the other, his previous statistics from the other league do not carry over. So if you hit 15 home runs in the AL, then get traded to the NL, you start over with zero. Your career stats are not affected, but for the current season your production in both leagues cannot be combined. Additionally, and much more commonly known, the American League allows a designated hitter, while the National League does not.
Major League Baseball, conversely, continues to promote the diversity of its constituent leagues and to operate much like state and federal governments: While states have a set of laws that must be abided by within their borders, an additional set of edicts must be followed anywhere in the United States. This is further reinforced by referring to the AL and NL not as conferences, but rather as leagues, though united under one larger governing body -- MLB. There is no better application of the federal and state government analogy than the existence of the designated hitter only in the American League.
The designated hitter bats in the lineup in place of any player on the field, but most always the pitcher. In the National League, where there is no DH, pitchers must bat. Since most pitchers are weaker hitters, they are almost automatic outs at the plate. This leads to more strategy in the NL: double-switches, bunts, stolen bases and hit-and-runs. In the AL, teams can worry less about manufacturing runs by falling back on the DH, not unlike Virginia football falling back on the TE Flat play every few downs. (Let's give it up for the return to Tight End U. this past weekend.)
Most baseball fans fall into one of three categories regarding the DH. One group is strongly for it; one group is strongly against it; and one group is still not sure how one league can have it while the other doesn't.
The designated hitter was an experiment for a few years in spring training, eventually adopted by the AL in 1973. That much we know. Further attempts to determine why only one of the leagues implemented the new rule and how the MLB continued to operate with such a drastic difference in the rules from one league to the other are like trying to find a book in the stacks: hopeless.
Maybe the best way to reconcile the difference is to accept that the two major leagues remain almost as independent as they were at their inceptions as competing leagues. For more evidence, think back to 1997 when interleague play was first introduced. At the time, the idea that teams from the AL and NL would play each other before the World Series was very controversial, a fact that is probably lost on many of today's fans who have since gotten used to it.
Additionally, in the other major sports leagues, winning your conference is not as big a deal as winning the entire league's title. But in Major League Baseball, winning the pennant has always been the benchmark for a great season. Since there were no playoffs other than the World Series until 1969, you really played the entire year to win your league, with a trip to the Series as icing on the cake.
As of 1969, there were still only two teams that made the playoffs from each league, increased to four each in 1994. With so few teams making it to the postseason, winning that triangle piece of felt was an achievement, showing you were able to take care of an entire league of teams. I think it's still impressive now because winning the pennant proves you are the best team in a league with different rules than the other. Just avoid trying to explain exactly why that is.