Is bariatric surgery the most successful way to keep weight off?

The sleeve, gastric bypass, the band, etc. Do people who have these procedures have a higher rate of keeping weight off compared to dieters? Supposedly, 80%-95% of people who try to lose weight with diets fail to keep most of the weight off long term. Supposedly, the numbers are about opposite for weight loss surgery. A lot of people will concede ”...for extremely obese people…who have exhausted all other means to lose weight…” weight loss surgery is acceptable. But, why not for people who are only 50 pounds overweight? Why do people have to be 40+ BMI or (35 BMI plus one pre-morbidity) to do the weight loss surgery thing? It works, why not?