This is a question with multiple reasons. The first and most common answer is that more and more people are losing their digestive health. Low stomach acid production is a vastly growing problem that results in nutritional deficiencies, and digestive disorders. When stomach acid levels are low, protein digestion is poor. This is what leads to the heavy bloating feeling after eating meat and can be stressful and uncomfortable. It also means that you will not be getting the energizing and nutritional benefits from eating meat.
The second role of stomach acid is to help stimulate bile production which helps break down food further especially fats. Animal products should contain healthy fats, but when not digested, they also just cause digestive issues. An example of an symptom of low HCl is the dreaded fish oil burp.
The second reason people see results on a vegetarian diet is reduced inflammation. This can occur because they are now eating more vegetables than before, eating less poor quality meats, or because they have eliminated foods they were intolerant to. Food intolerances occur when your body starts to develop an inflammatory immune response to the proteins in the food you’re eating. It can be any food, and doesn’t always have to be proteins. Many people have intolerances to fructose which is a sugar found in fruits and veggies, but in higher amounts in sugary foods and beverages. Food intolerances usually occur after the digestive system becomes compromised. It has nothing to do with whether or not the food is good or bad for you. If someone has an intolerance to chicken because they have been following a low fat diet too long and switches to a vegan diet with no meat, they will see the benefits of removing the chicken, even though they didn’t need to remove the rest of the meats.
The third reason is the consumption of more cruciferous vegetables and fruits with antioxidants and fiber. Antioxidants and fiber are two of the most important nutrients for vitality and weight loss. Eating more green vegetables and high antioxidant fruits like blueberries is hard to argue with. These same benefits would be seen though even if meat was not removed from the diet. Fiber is critical for digestive health, detoxification, and help control the digestion and absorption of carbohydrates. This is why most research on diabetes finds that that switching to a vegan diet provides similar blood markers to a typical diabetes diet over short periods of time.