For one, meats can be bad for our health. Regularly eating processed and red meats has been linked to chronic diseases. (Recently Harvard researchers published a study in the American Journal of Clinical Nutrition that shows an association between eating red meat and being more likely to develop diabetes.) Plus pound-for-pound, we get more antioxidant-type vitamins (like Vitamin A and C) from fruits and vegetables.
Four, it takes less water, sun and even soil to grow plants versus raising animals for meat, which means it's better for the environment. Finally, if you're a restaurant, it could be better for business according to this NY Times article about Aspen's adoption of Meatless Mondays.