For the five years I attended Roskilde, I stayed at GAT, mainly because I never arrived earlier than Tuesday or even Wednesday and I was afraid not to be able to pick a decent spot for my tent. Also, traveling by plane, it allowed me to pack lighter - although I brought back a couple of those orange tents over the years.
For my first two years ('11 and '12), I remember GAT was on the other side of the eastern entrance, across the road, which meant quite a long way to the tent and back - better not forget something in the tent. The GAT area north of the skate park in 2013 was a great improvement, as it was much closer from both the festival area and the main camping. The current location is just perfect in this regard, and it is still quiet enough to take some rest when needed - at least while the scorching sun doesn't turn the tent into some kind of sauna, but well, one can always bring a sleeping bag under the trees. But those "perfect" areas come necessarily in limited supply, so I guess it is not possible to expand a lot those options. Also, the GAT / tenthouse entry ticket is larger than the regular camping, but this probably doesn't imply that it brings more revenue to the festival, even if demand is huge, as these areas are sparcely populated, compared to the main camping. Also, the tents have a cost, and it takes some manpower to pitch them, even if this is done by volunteers.
bolstfu wrote: Why the waiting list, then? To check the demand for next year, maybe?
I think the waiting list is related to the ticket exchange, which is linked from the ticketmaster web site (follow the "Sell your ticket safely and legally here" link.