Pure vegan diets work for some, yet lead to feelings of constant hunger, weakness, lack of energy, sleepiness or headaches in others. As a result, people who try them frequently revert back to their old ways with vengeance when they need that extra boost of energy, and yo-yo dieting usually leads to worst outcomes than no dieting at all.
If a pure vegan diet worked for you and your gut bacteria, that's great. It's a challenge for a lot of others.