At this stage of the decline of the JW org it would make sense to have as few GB members as possible.
First, in answer to the why of your question; the old guard; the generation of 1914 who would "never die in this system of things"... have all now died. This is completely in keeping with the historical truthfulness of the Watchtower predictions and has left a team of seven zombies who are determined to keep alive the sacred cow which was the Watchtower organisation.
Second, it is easier to control and reach agreement with seven minds than it would be for twelve.
Third, the chances now of new untested members awakening to TTATT are very high. Such a situation would rapidly destabilise the already weakened organisation and be a trigger for the rank and file to follow suite.
Fourth, why share the privileges and financial perks with more freeloaders and thereby dilute the benefits?
Having said that it is clear from the collection of 'Santa's little helpers' that in the event of the death of the present crew there is no shortage of wannabees.