I may be wrong here (I'm no biologist), but here's my theory:
While we have evolved in certain ways, to cope with mathematics and such, our brains still function very simply, like cavemen.
So picture a caveman, eating regularly (a few times a day say). His brain recognises this, and realises that if he's eating regularly, there must be plenty of food about. So your brain sends out signals to make you want to eat fatty stuff, in order to stock up food in your body (i.e. fat reserves) for when the food isn't so plentiful. Like squirrels saving for the winter. The problem of course is that right now, there is plenty of food available because we have McDonald's and Tesco etc. The fruit and veg stuff is nice, but there are two tihngs about this:
1) it isn't high fat, so isn't good for storing reserves
2) if you eat something that you don't like, then by eating it regularly enough, you'll grow to like the taste of it (hence how babies and kids like stuff they didn't at first). This is again your brain at work, because if you're eating something a lot of the time, your brain assumes that that's all that is available to eat right now, so you'd better get used to it and like it. Now, we eat fatty foods probably just as regularly as low fat foods, and if anything perhaps pizza, chocolate and the like a little bit more. so by eating more of it we grow to like it more, which means we eat more of it, etc. a viscious circle.
The sangri-la diet exploits this to its advantage.