I have to agree that genetics are a factor. It's much harder for some to lose weight than others. However, environment plays a big deal as well.
If a person trains hard enough/long enough and creates a caloric deficit they can get down to the body fat percentage they're targeting. You're simply lying to yourself if you do not believe that.
Historic / scientific evidence demonstrates that ANYONE is capable of being sickly "skinny". I didn't say fit but skinny. No one is fat in a concentration camp if they've been there over a year right?
It's an extreme example but it demonstrates that "yes it's possible". The counterpart to that goes back to discipline. While it is theoretically possible to work ANYONE in to a fit and trim shape. They may not be able to mentally sustain the rigors and the length of the program and slack or give up. We all know what I'm talking about – there's always been a second here or there where you could have pushed just a little harder picked your feet up just a little higher. It's the person who at that moment drops down and pushes 20 and is willing to endure that will hit that targeted goal no matter what the genetic predisposition says.
Just to qualify.. this is coming from a guy who's boxed, wrestled and done martial arts from the age of four. I know what it's like to train 14 hour days months at a time cut weight, add mass consume over 10K calories a day and still have 4% body fat. Heh not now back in the day :)
It's all about heart. A lot of people don't need to push their bodies that hard nor do they want to, so they just don't. It doesn't make you a bad person it just means you have other priorities. It could take one person a month to get in to a certain shape and 2 years for another person. It doesn't mean it's not possible. You just have to want it more than the d-bag that has it easy.