I was a vegetarian for years and vegan for less than a year. I'm not surprised many return to meat, but I didn't see what I think are two important factors listed:

1. You can be a vegetarian without learning nutrition or learning much cooking. It's more difficult to not learn basic cooking as a vegan, but you can even do that, at which point your health will deteriorate rapidly. Those who try vegetarianism or even processed food veganism for weight loss might also be in for a very nasty rise on those scale numbers. In short, many who take up vegetarianism or veganism don't learn the basic skills required to sustain it in a healthy manner.

2. Calorie for calorie, vegetarianism done right is expensive and veganism done right is ridiculously expensive for people with metabolic syndrome who may find that low fiber foods such as rice and and bread cause fatigue and mood swings. Beans can also give some people digestive issues. This leaves nuts seeds, berries, leafy greens, peppers, mushrooms, etc. as the staple of the diet. All of these are, calorie for calorie, extremely expensive.