Top down you'll get studies with graphs of veg intake vs "illness" and pretty much the more veg you eat, the better. This is an outcome based "why" not a biochemical based "why".
Bottom up there really are no completely ideal foods, but there are foods that specialize in certain things, and much as animal meat specializes in protein, veg specializes in fiber and the micronutrients (vitamins, minerals, etc). This specialization is not absolute. You could get your vitamin A from a meal of liver once a week or so, but it seems easier just to eat a carrot every day or two. Unless you really like liver. Fruits are almost as good as veg other than having lots of sugar.
The "why" for the hair shirt crowd is its pretty easy to F up vegetables such that they taste horrible, therefore they feel better because you're suffering, because you deserve it. Why? Doesn't matter much. They'd probably be really pissed off that I actually enjoy the taste of a salad of proper proportions, or a snack of eating a carrot or whatever.
You guys in the UK are pretty lucky to have science based guidelines, in the USA our govt enforced guidelines are based solely on who paid politicians election funds, so we're stuck with "eat as much grains as you can" and stuff like that, designed to make us fat.
