Here is what Blomkamp has to say about why the prawns haven't turned on the humans in the 20 years they've been on earth. i'm glad to hear it cause it was a big question during the movie.
The hive mind [concept] is the most important thing to me, because I love the idea of a civilization that can build all of that technology and then, at the same time, just have a massive population that was just drones that needed direction, and were absolutely incapable of building that stuff on their own... individually, they may be feeling oppressed, but they don't have it together enough to form a resistance and back one another.
(I had been thinking that the prawns were just so hungry that they traded all their weapons for catfood. i like this better.)
