People are different in all ways. Unfortunately, Some of us do not have a high enough metabolism to burn off what we eat fast enough. Other people may have a medical condition like hypothyroidism which can contribute to being overweight
I don't think it is hard to lose weight. It all depends on the individual's determination. I lost 30 pounds on my own and eventually joined weight watchers and lost an additional 30. Anyone that really wants it can obtain it.
The main reason would be because of a slow metabolism. People with this disorder need to eat healthier and stay active more than others. It's unfair, but by doing so, you will see a much more result in your body by doing so.