In 1997, the Milwaukee Brewers finished 78-83, third in the American League Central. In 1998, the Brewers finished 74-88, fifth in the National League Central.
The Brewers switched leagues in 1998 when Major League Baseball expanded from 28 to 30 teams. The prospect of having two 15-team leagues was apparently never discussed, despite the fact that interleague play had been introduced the previous year. Interleague games could have been scheduled on a daily basis, rather than having dedicated periods of interleague play, with one leftover NL matchup. But no, the NL added two teams - one of which was the league-switching Brewers - and the AL stayed at 14.
It doesn't require a genius to figure out what happened. Commissioner Bud Selig, heretofore the Brewers' founder and the guy who restored the big leagues to Milwaukee in 1970, had been a huge Milwaukee Braves' fan between 1953 and 1965, when they left for Atlanta. Heck, he'd been a minority stockholder in the Braves, and had attempted legal action to keep them from leaving. Yes, he had succeeded in bringing baseball back to Milwaukee, but had failed in keeping his first love, the NL Braves, in town. Arranging for the Brewers to switch leagues was a way to erase that failure from his record.
Selig is no longer the owner of the Brewers; Mark Attanasio is. Would switching the Brewers back to the AL really be that big of a deal? It would level the field as far as scheduling goes, and switching a current AL Central team to the West - the Royals, for instance (longtime members of the AL West from 1969-93) - would create a pair of 15-team leagues.
I doubt a move like this could take place while Selig is still the commissioner, but at some point he'll step down. It's all about fairness, really, and the current set-up offers inherent unfairness between the AL West and NL Central. The Brewers' only World Series appearance was in 1982, representing the AL. Going back to their league roots seems like the right thing to do.