As a clinical therapist with over 20 years of counseling experience, and a newly sleeved person, I spent over a year researching procedures before actually getting the surgery. I am finding that what are often presented as scientific facts is more like a hypotheses of how surgery works. The surgeries used are still in their infancy and much is still unknown about their processes. When I read some of the other members posts on this forum, it becomes obvious that misconceptions about weight loss surgery abound and that we must do a better job of equipping surgery candidates for the long term change necessary for successful maintenance.
It takes a combination of so many things in order to achieve sustainable weight loss, no matter which method to lose weight is chosen. Personally, after having the surgery, I believe that most people are more likely to revert back to old cues in their environment, emotional triggers and biological "set points" without guidance and serious effort to change. Working on those things for myself right now. Hopefully more research will provide answers that can better predict one's genetic predisposition and make more tools available for anyone with obesity and its comorbidities.
Given all the variables, it is important not to lay blame upon the persons struggling with their weight loss, until more information is available. Making a blanket statement about what causes weight regain is making assumptions which do not apply to the whole population and may not be rooted in the scientific facts. Biology is a powerful thing which wants to maintain homeostasis. Hormonal influences just may be at the heart of the matter.
Best of luck to all of us in reaching and maintaining our goals.