Culture - our culture includes the idea that prosperity is, in part, expressed through diet. Meat has been adopted as part of the way we express our 'middle classness" or our success.
Habit - that culture has become habit. We've been taught the "meat and veggies" food pyramid. We grew up with it in every school cafeteria and public restaurant. There have been millions spent in ad campaigns. Where's the beef? It's ingrained in how we think about meals.
Industrial agriculture - it's easier and cheaper to buy a hamburger than to get a plate of fresh veggies. You get more calories for your dollar. The cost of meat is subsidized. It's more convenient. Our mainstream grocery stores are set up to promote the purchase of meat.
We need a return to local agriculture, education about better diet, involvement in our food systems.