It used to be that veggie burgers were just for health fanatics and those who did not want to eat meat. Maybe they didn't want to kill animals, or maybe they had religious objections. The truth was that there was a very small percentage of people who didn't want to eat meat and an even smaller percentage of those people who wanted to eat veggie burgers.
The old veggie burgers, the ones that you found in California during the 1970s were not the best tasting. They were made with lentils or sprouts. But that changed. Over time people began to take more creative routes with their veggie burgers. Now you can find them in big chains that normally would only sell beef burgers.
Another move that shows that people who normally would scoff at veggie burgers are involved in creating really gourmet veggie burgers. Just recently the New York Times had a big piece on veggie burgers.
You have celeb chefs and other chefs who are in the kitchen at really nice restaurants and they are involved in making top quality veggie burgers. They might use mushrooms, lentils,quinoa, brown rice, or black beans. The choices of ingredients is endless, all it needs be is non-meat food.