Michael Pollan notes that the shift away from Omega-3s is primarily associated with the shift from consumptions of leaves to seeds (by both humans, and the animals eaten by humans), and that this shift may explain most of the health problems caused by the Western diet. The issue then arises as to why would humans prefer a diet which is manifestly unhealthy? Susan Allport in the “Queen of Fats” notes that populations will tend to shift to foods with less Omega-3 and more Omega-6 if given the choice. She suggests that the reason is the desire (whether conscious or not) to reduce the rate of metabolism, and reduce hunger pangs.
In my personal experience, I think this explanation is too subtle, since the reduction in metabolism is probably a long term side-effect of eating seeds, that most people would not immediately be aware of, and thus would not be as significant as factors which are more immediately obvious. For me, although salads can be tasty, grain products are obviously more satisfying and filling than greens. Greens have a much shorter shelf life and require refridgeration. There is a strong cultural bias (linked to our paleothic past?) towards meat and bread (vegetarians are wimps). In addition, fish are the mainly remaining animal food that eat wild greens, but most fish are either too bland (sole) or too fishy (sardines). Generally speaking, processed grain foods are cheaper and more convenient to prepare, and have the attraction of added sugar.