Not eating flour?

I would like to know the main reason why we can’t eat some kind of flour, even if its not white flour, for me being vegetarian, it makes it very restrictive, I don’t eat lentils and beans, and no meat so it makes this protocol neally impossible for me to stick to with out the flour, I can do the sugar free and and I can do the fasting, I have attempted restrictive diets like this before and I couldn’t stay on them, and went back to eating as I did before. I don’t want to quit before I start, I see some great ideas and suggestions in what I’ve read so far.