There are a lot of different factors.
Sometimes it's time. When I'm crunching at work (working minimum 12 hour days) I don't have time to workout and I'm probably sitting most of the day. I'm also getting fed whatever I'm given and that isn't always healthy. While working on my last project, I gained a lot of weight because I crunched for well over a year, didn't eat healthy, and didn't work out because I didn't have the time or energy.
Sometimes it's lack of willpower. I know I should eat healthy and I do generally try (I'm down a total 8 pounds in the last month :)), but sometimes you just want that hamburger or whatever unhealthy.
Sometimes I don't have time to cook and have to eat out or order. I once listened to Alton Brown speak at a book signing and he had a great quote when someone asked him why we're fat-- "American isn't overweight because of what they eat at home." When I cook at home, I generally cook on the healthier side (though not all the time >_>) but when I eat out, I don't always have control over what I get and how healthy it is. It's all overly salty huge portions with who knows what in it. At least I've gave up fast food long ago.
Sometimes I'm just lazy. There are days I wake up and I know I should go to the gym, but I just don't want to.
All in all, if you're overweight, it is difficult to not be overweight. There are so many factors that lead to you just staying the same or getting worse and it's an incredible uphill battle the whole way to get back down to a good weight. I would love to lose another 10-15 pounds, but it's difficult and most of it just me. I'm getting there, but it's hard work.
Six months ago I was pushing 230 pounds, today I'm at 214 so I'm getting there. 
