Having been involved in judging a ute if the year test a few years back I can categorically say that we were in no way forced or directed to pick any vehicle. It's nice to throw around conspiracy theories that it's all rigged but the mags would not last.
That said it does come down to vehicles that are and aren't supplied by the manufacturers. They are all invited but for various reasons they may not accept. Press vehicles may not be available etc
Also in many cases it is only open to new or significantly updated vehicles so the competition isn't as comprehensive as some would like.