The study that you linked says that 20% of dieters lose significant amounts of weight and manage to keep it off long-term. Read the summary section:
SUMMARY
Results of random digit dial surveys indicate that ≈20% of people in the general population are successful at long-term weight loss maintenance. These data, along with findings from the National Weight Control Registry, underscore the fact that it is possible to achieve and maintain significant amounts of weight loss. [...]
http://ajcn.nutrition.org/content/82/1/222S.full
The NWCR cited in the study is here:
http://www.nwcr.ws The NWCR is a good place for weight loss tips and research findings on the best ways to lose weight.
As for why some people (20%) can lose weight and keep it off, I think it comes down making weight loss a top priority in your life.
A lot of people make an effort to lose weight, but they only attempt it haphazardly. They engage in fad diets; or they diet hard-core for a while, lose some weight, and then go back to their previous bad eating habits and regain the weight. These are the unsuccessful 80%.
Meantime, there is one segment of the population where people have almost 100% success at controlling their weight: People who spend their lives in front of cameras or on-stage: Actors, announcers, politicians, singers, newsmen, etc. Why are they so good at weight control? Because looking good is their top priority in life. Their appearance is their livelihood. Check them out in the off-season or between gigs, and you’ll see that they often gain weight. But when it comes time to get in front of a camera or go on-stage again, suddenly their weight is back under control.
So it’s my opinion that if you want to be one of the 20% successful crowd, you need to prioritize weight loss like your livelihood depends on it (IOW, like you were an actor or politician or newsman): Research the subject properly; use safe, effective, proven weight-loss techniques instead of fad diets; and make long-term changes to your diet and exercise instead of just dieting hard for a while and then going back to previous bad habits.