Major League Baseball last expanded in 1998, when the (then) Tampa Bay Devil Rays and Arizona Diamondbacks were added to the group, making a 30-team league.
That’s quite some time in the expansion era; since expansion of baseball began in 1961, the previous longest gap between expansions was 16 years, between the addition of the Toronto Blue Jays and Seattle Mariners to the American League in 1977 and the creation of the Colorado Rockies and (then) Florida Marlins in the National League in 1993.
Among other things, adding two teams to MLB would make scheduling a lot easier; two 16-team leagues break down easily into divisions of four or eight, and there have been numerous proposals of how to schedule such a league that would help (among other things) making up postponed games.