The permaculture garden uses more water than would drop there in normal rainfall. And it requires heavy inputs of organic matter from off the farm, at least in the first years of establishment; that's how the crappy piece of land becomes lush. And that water and organic matter are produced and transported by use of large amounts of fossil fuels, so the indirect consumption and pollution are much higher than suggested by permaculture adherents - again, at least in the years of establishing the place.
In this respect it's like renewable energy. We can't get silicon wafers and aluminium and concrete and steel to make solar PV and wind turbines without releasing CO2 (concrete setting releases CO2) and burning lots of fossil fuels with vehicles tooling around digging in the ground mining, refining, constructing, maintaining and so on. Now, this is low-carbon - solar PV/thermal and wind are about 0.07kg CO2-equivalent per kWh vs 1.50 for coal - but it's not zero-carbon, as commonly claimed.
However, the major issue is the labour and skills involved. Typical Australian household spending on food - including booze and eating out - is about $240pw. Obviously we can get by on half that, but let's take it as it is. Minimum wage here is now $18.93ph or $719.40pw before tax, and though few households with just one person on minimum wage would spend $240pw on food, it's still just 12hr40' of work to pay for all that. So if you have to spend more than 12hr40' pw on growing your own food, then it's simpler to just go and get a job and buy the stuff - from those much more wasteful big farms and supermarkets and restaurants.
Now add in the fact that you actually need some skills and knowledge of the local climate and conditions, and that these will take some years to achieve, and you can see why people just go to the shops instead. Obviously, working in the open air every day among nature has other physical and mental health benefits, and people often choose to do things that are "not optimal" because they are optimal for them. But as cool as it is, for these reasons I don't see it becoming widespread.
