Reorganization of 1963
The current minor league structure is largely based on a significant reorganization that occurred before the 1963 season, which was caused by the club and league contraction of the 1950s and early 1960s. In 1949, the peak of the post-World War II minor league baseball boom, 438 teams in 59 leagues were members of the National Association of Professional Baseball Leagues. By the end of 1963, only 15 leagues survived in the United States and Canada.
Read more about this topic: Minor League Baseball