The nutrition habitudes change. Most vegeterians do it because of their philosophical believes; they know how precious a life (you name it; soul or intelligence) is and they don't want to kill animals. Our technology allows us to stay alive without eating animals; we can produce vegeterian meals that taste almost exactly like beef, and with exactly the same nutritive (vitamines, proteins etc) composition as naturals.
In time, we'll understand that killing animals for nutrition is no different than slavery or colonization. We'll be ashamed of our past.
The next step of this will be to become autotrophic. We'll figure out that killing plants is also a sin! And as our technology allows it, we'll start injecting chlorophylles into our body and just like plants, we'll start producing our energy by ourselves and working with the solar power. This will allow us to stay alive without killing any living and this will be very cool! If no one does, be sure I will!!!
Challenge Emre Sokullu to a bet on this prediction!