Actually, ever since 2005 (or thereabouts) the population of Australia has more overweight and obese people than the US. It's not just America - it's almost every developed nation except the French and the Japanese. It tends to be worse in Australia, New Zealand, The United States, Mexico and Cananda.
There are actually many reasons:
1) Increased sedentary lifestyles - technology has made our lives easier, although many people don't realize it. Most of us have to do a fraction of the physical work our fathers and mothers had to do - whether in the workplace or in the home. Desk jobs, cars, wide-range public transport are all good things, but they also mean that we don't move around nearly as much
Also, in the past, community events, interactions, parties, hiking and travelling were much more popular because... well, frankly there wasn't that much to DO at home. Home was eating, sleeping and reading - but most of the population went outside for their fun and games. In the past, "go to your room and stay there" was a real punishment, since kids liked to play outside and would always resist being called back into their home at night. But now, with the internet, TV, music players, video games - you can have all the entertainment in the world at your fingertips in your house and you don't have to even get off the couch. Want to talk to your friend? Call them from the couch. Want to know what's going on outside? Turn on the TV. Want to feel an adrenaline rush? Watch an action film or play a game.
2) Food is cheaper and more widely available than ever before. Now, a lot of you are looking at your grocery bills and are puzzled by this statement, but that's because you're not taking into account inflation. Food, at least in the developed world, is available to almost anyone. Few people go hungry at night, unless they are in the most dire of situations (homeless, for example). Virtually no one dies of starvation in the developed world - not even the very poor.
But try telling that to our bodies - you see, we evolved on the Savannah plains, where food had to be caught or harvested and was hardly secure. Food had to be eaten there and then, because our ancestors didn't have farms when they evolved, they didn't have refrigerators. Sugary foods and fruit in particular had to be eaten RIGHT THEN AND NOW - your body wanted you to eat as much of it as possible, as soon as possible, because chances are it wouldn't be there tomorrow! The body was designed to cram a lot of food, go hungry for a while, then cram a lot of food. Not exactly healthy, but that's how our bodies worked, because that was the conditions our ancestors grew up in.
But we don't live on the Savannah anymore. Sugar and Fats are widely available. Yet, our bodies don't evolve nearly as quickly as our societies. Our bodies still compel us to cram sugars and fats because our bodies still perceive these foods to be rare, useful treats.
3) The type of food has changed. We get a lot more sugar and fat then we need to. Our bodies find these foods delicious because they are high in energy. Back in ye olden days, such food was rare. Sugar was found almost exclusively in fruit, and fruit wasn't exactly in abundance in Northern Africa, where humans are generally thought to have evolved. But now sugar is everywhere. It's in everything. Sugar taken moderately is nothing to fear - but US food companies knew that sugar made things tasty and that tasty things sold better, so they put it in their products. Sales are everything to companies.
4) We don't like to cook. Our lives are incomparably easier than our ancestors, but we don't think so. We're used to the technology that makes our lives a BREEZE compared to the life of a 13th century English Peasant. We take it for granted.
We have more free time but we also want to spend that time on entertainment. Cooking takes time. Even the simplest cooked meals can take up to an HOUR to cook! Hell, I had to go to work from 9 till 5, and I want to watch my TV and I don't want to spend an hour in front of a STOVE! So the thinking goes.
So, people in developed nations buy pre-packaged food. These foods aren't necessarily awful, but they often are high in fats and salts and sugars. They are high in these ingredients, first as a sales-tactic to increase sales because hey, people naturally like such foods, but also because high sugar levels actually preserve food. Bacteria does like sugar, but not too much sugar, and they really don't like salt.
Cooking foods is incomparably healthier and can be tastier, but requires a lot more effort, and we don't want to put that effort in. I know I can't be bothered cooking.
There are many more factors as well - advertising, body weight issues, poor parenting, the body's natural tendency towards homeostasis and the alterations of your appetite due to obesity - but it's actually not a simple topic.
As for why fat people find it so hard to lose weight - it's because once the body has reached a certain weight, it fights any attempt to reduce that weight. For someone who is obese, the body and the brain adjust to that overweight size as the normal baseline. It will fight tooth-and-nail to prevent you from losing that weight because on the Savannah plain, losing weight on purpose was suicidal foolishness! "What?!" your brain screams, "You want to LOSE weight!? What if there's a lion around the next corner? What if there's a drought tomorrow?" Never mind that most of us don't live on the Savannah, or in drought prone regions, and that the supermarket won't disappear tomorrow - our instinctive part of our brain doesn't know that. As far as our instincts go, we're still on the Savannah, fighting for food every single day. The body doesn't want to ever let weight go.