PR literally had nothing going for it in terms of attracting the GA. The trailers weren't impressive. They showed lumbering giant robots punching big ugly monsters in nighttime scenes. They didnt emphasize the human relationships, emotions or humor to bring in females/family They didnt have a recognizable actor or director with crossover appeal. There were no money shots that would make people go "whoa i need to see that". Robots vs monsters is a very nich concept with a small fanbase, and WB did a terrible job with marketing.
Say what you want about Transformers but those guys did a masterful job advertising those movies. They put the cool cars, the hot girl, the funny lines, the epic scale and the sam-bumblebee relationship at the forefront. And plus they had a notorious director at the helm. Look at this trailer:its like a 100times more exciting than anything PR put in its trailers yet DOTM is a worse movie.
http://www.youtube.com/watch?v=kHRf01Gjosk
Don't blame the GA for Pac Rim failing. Blame WB for not giving them enough reason to go see it.