I've just watched this on iPlayer. I think I got a better understanding of how people can end up in that kind of situation; though that might also be due to some informal studying I've been doing lately.
I think there's a lot of research/understanding that still needs to be done about the causes of morbid obesity and the influences of genetics, environment, psychological predisposition, and the effects on metabolism.
It takes a lot of calories to support a large body mass; if morbidly obese people go on a severely restricted calorie diet they can exacerbate their medical conditions or introduce new ones - I'm thinking about ketosis in particular, though I'd have to read up more about that.
I don't know enough science about metabolism, but one interesting (and horrifying) episode in recent history did shed some light on how people can end up with drastically altered metabolisms: en.wikipedia.org/wiki/Dutch_famine_of_1944. Those who survived the famine had the capacity to hold on to every calorie they could get hold of, but it was the effect on those who were in utero at the time that was startling, because they also developed the capacity to hold on to every calorie they could get hold of. That wasn't a genetic factor, but a response to an extreme environment that has had a knock-on effect on subsequent generations, but which will eventually fade away.
Another bit of informal studying I've done highlighted a situation that occurred further back in history, about the slave trade, and how many of the enslaved people who survived the privations of the slave ships came from areas of Africa that had low levels of salt and low levels of fat in their diet. They were genetically perfect for life in their original circumstances, and that probably helped them and their offspring to survive subsequent poor nutrition. Genetic factors don't go away as easily as environmental effects and once people with highly efficient calorie- and salt-conserving abilities get access to high fat and high salt foods, their metabolisms can operate to their disadvantage.
Then there's stress and depression. One symptom of depression is that some people eat more and some people lose their appetite. Another symptom of depression is that people withdraw from normal life and don't go out much. The morbidly obese might not start out withdrawing from normal life, but often they end up doing that because of mobility problems or not wanting people mocking them. It can end up as a vicious circle.
As for "enablers", I must admit I've wondered about that before. But going back to the high calorie needs of a larger body, and also the depression often suffered by morbidly obese people, I think any carer of a morbidly obese person would have to be heartless and possibly dangerously wrong to significantly restrict the diet of the person they're caring for. And psychologically that could be devastating - these people have already lost (for whatever reason) the ability to live a normal life, so taking away the little control they have could be the worst thing to do to them.
It's already been noted that people on restricted income and with restricted mobility, don't really have much choice other than to go for the cheap'n'easy options, and those are most likely to be the high salt/fat/sugar options, because that's what sells, so they're the "pile 'em high, sell 'em cheap" types of foods. The programme showed a packet of a soup in a cup - cheap to buy and easy to prepare but less likely to be as satisfying as home-made soup (physiologically and psychologically). I wonder how many people think that such a cup of soup might be a good thing for weight-loss purposes.
I learned from that episode a lot about the people who were brave enough to be seen at their most vulnerable and tell us their story. We really do need to listen to them, because they're the people who can inform us the most about the human metabolism in our modern world.