I think the key factor missing is education. While there's tons of diets out there, there is also tons of studies that show tha most of those "diets" do not meet nutritional needs. Many are very hard to follow, resulting in frustration and people giving up. Proper nutritional education is key for any kind of success. Personally I'd like to see this information incorporated into the school education system.
Also, even though there are lots of gyms available, as well as other forms of exercise, again it largely comes down to education. I've been to the gym where i regularly see people who aren't really working out, or are using machines improperly. this will cause injury, and won't help wtih weightloss at all.
When I've travelled through Europe and parts of Asia, I found it was very easy to get healthy food choices on the go. In Italy, there was fresh fruit being sold along the streets. In France, the corner stores were filled with fresh fruit, veggies, fresh breads and cheeses. In Thailand it was easy to get a veggie stirfry or freshly squeezed juice - as it was sold on many street corners.
In North America what do we have on the street corners? Hot dogs, sausages, maybe chips and pop. But you'd be hard pressed to find fresh fruit and veggies. Even our convenience stores are filled with crap. There's very little options that are truly healthy for people on the go.