Normally I leave the diet posts to Warren. He's spends way more time and energy on it anyway, and he is far less inclined to indulge in unhealthy non-paleo cheats than I am. On the other hand I'm the one who is about to be nursing two kids at once, so I do have opinions.
Brains are made mostly of fat. Human babies have big brains, and for the first few months after birth those brains are going through a process where the insulation on all those neurons is being built, and that insulation is pure fat. It's not just any fat either, it requires a lot of Omega-3 fatty acid to build. Where does this fat come from? Well, if the baby is nursing it comes from the Mom, and if the mom isn't getting enough of the right kind of fats in her diet, then her body takes what it needs out of her in order to feed the baby, including shrinking her own brain to do it. I'm pretty attached to my brain and more than a little vain about it, so I don't want that happening to me.
So, most women have a baby, and then they have all this extra weight they want to get rid of so the diet. This probably means they follow the typical dieting advice and avoid fats, and when they do get fats the ratio of Omega-6 to Omega-3 is way too high because modern agro-business feeds cows and chickens so much corn, and pregnant & nursing women are often discouraged from eating fish because of concerns over mercury. (When I was in the hospital with Duncan I flat out lied about who the swordfish dinner was for, because otherwise I'd have been stuck eating their lean crap.) If a woman is starving herself of what she needs, either because she is trying to lose the baby weight fast or simply because the current western diet doesn't have enough of the right kinds of fats it's no wonder that she might be prone to depression, especially once you throw in the sleep deprivation and general relentless nature of caring for an infant. Personally I'm a lot less concerned about how soon I fit back into my old clothes and a lot more concerned about giving my baby and myself the nutrition we both need for our brains.