I know that minor league baseball teams/leagues used to be classified as “D,” “C,” “B,” and “A” ball. When and why were the designations changed to “A,” “AA,” and “AAA” ball?
The system as we know it today was adopted in 1963. The classification system was set up in part to set prices for major league teams to pay minor league clubs for acquiring players. An “A” player cost more than an “E”.
For many years, the Pacific Coast League was “Open” and its players weren’t subject to the major league draft of players. That changed when the Dodgers and Giants moved west and the PCL dropped to AAA.
In the late 50’s and early 60’s, the minor leagues were decimated by a loss of revenue to televised major league games and loss of populous territory to major league expansion and relocation. Most of the lower-class leagues just up and folded, so the majors decided to just get rid of the B, C and D classes and classify the few survivors as “rookie class” leagues.
And to add something else, today’s divisions are strictly regulated by Minor League Baseball, aka National Association.
Each level has certain standards for its facilities, rosters, and markets. A ball is also divided into “high” A and “low” A. There are age requirements for the rosters for each type of league. Most Rookie leagues and low “A” don’t start play until after the draft, when all the new free agents have been signed.
There are also independent minor leagues like the Northern League, which operate outside the classification system and are not subject to the rules of MLB.