I worked with Geoff at his farm in AU, and he's done some amazing work there. His swale and pond layout, as well as the food forestry corridors for his animals are spectacular. Large scale implementation of arid desert techniques from the Permaculture handbook is one thing that definitely needs to be looked into.
I would probably guess a lot of reasons. I'm not sure that there has been "that" many large agro studies on many of the techniques. Also, I think something as simple as planting large crop on swale (not in a straight line) drastically changes how the farm would run (what equipment to use, industry-wide crop measurements, etc). That's just the first thing that comes to mind.
