Just a random guess, but my bet is that it has to do with logistically, Single A participant numbers barely being able to field enough guys to fill every weight bracket in a 32 man format, as Single A, by virtue of there being less kids generally, has far fewer wrestlers to field full teams, etc. There must be bracket uniformity participant-wise, at some level, in order to not have weirdness and incongruity when having top 8 place in unfilled 32 man brackets.
My thinking is that perhaps top 6 placing was the highest volume of placers, for the numbers across the board in each disparately filled/unfilled brackets, to ensure total uniformity with how many bouts are needed to win in order to place, etc.