1. One reason may be that type o people who are confirmed vegetarians are upset about the recommendations of eating meat. I've come across people like this. They won't even look at the AVOID foods.
2. After having been involved in the post-polio movement for over a decade, I'm convinced that some people don't want to get well.
3. MONEY. It takes effort and money to seek out all the non-wheat, non-dairy and non-nightshade family type of foods. If you are very poor you will go for the cheapest foods. Look at what's available at foodbanks! It's usually wheat, tomatoe and potatoe products.
4. TOO SICK. Someone who is so sick that they can barely function, might find it too exhausting to cook from scratch and to read labels of everything.