Although the USDA has limits on the amounts of pesticides which are safe for human consumption, those limits are created assuming that the pesticide currently being tested is the only one you eat. With non organically grown foods, most people consume a hodgepodge of chemicals which have not been tested together for safety. For your health. Tons of studies have linked consumption of red meat to increased heart disease. For your wallet.
Vegetarian items are almost always cheaper than meat.
Let's face it, preparing raw meat is pretty gross. Beans are much nicer to deal with. I plan to cook only vegetarian when I cook at home. When I go out to eat, I may eat meat. If I'm in a group of friends ordering pizza or Chinese food, I don't want to limit everyone else's choices and have them annoyed with me.