Actually I do know why our group reservations have less priority to be placed together than the Marathon Prevost groups (and other large groups). It is the type of group reservation we have. Ours is set up where we have so many sites blocked out and each individual person makes their own reservations directly with the Disney Group Reservations office with individual cancellation policies on each site. No one person is really responsible for the money.
However, there is another type of group reservation where one person calls and says I need "Loop 700" for x number of nights and we want these benefits, then Disney gives them a price. Then that one person is responsible to pony up the money whether they get the convention booked or not. THAT's how you get a loop blocked off. Since none of us are willing to take that type of risk and there are probably additional fees with that, then we go with the less exclusive group reservation. They actually do try a little harder to get us together than they would with just general requests or Grand Gatherings, but still no guarantees.