The explication to your question is multi-faceted and not so easy to understand if one is neither Roman Catholic nor understanding of the nuances of religious life (Trappist).
The Trappists were founded in 1664 as a branch of the Benedictine Order at the La Trappe Abbey. The abbot of La Trappe has a greater authority of jurisdiction than the other trappist abbots. Under canon law this is permitted because La Trappe is the motherhouse of the Trappist Order.
The history of the Trappist Foundation in 1997 is of a totally different perspective. Originally only La Trappe Abbey could grant the title of the label to those Trappist breweries which could safeguard the integrity of the Order. All other monasteries which could not or would not abide to the conditions to be met, would not be granted the Trappist Label.
Since 2012, four other Trappist monasteries have been added to the list, including St. Joseph's Abbey in Massachusetts, United States.
The three basic rules for granting the label are:
- The beer must be brewed within the monastery, by the monks or at least under the supervision of the monks.
- The making of ales should be of a secondary importance to the monastery and should be seen as a witness of monastic life.
- Any profits from the brewery are to be use to help sustain the monks in their life of prayer. Any extra income is to be given to some sort of charity.
There seems to be no other Trappist Abbeys that were producing beer at the time the label was founded. Since abbeys are independent of one another, sources of revenue are not limited to the brewing of ales. Some abbeys made vestments, while other distilled products such as kirsch, as was the case at Fontgombault Abbey from 1849-1905. Fontgombault Abbey (Wikipedia)