Too much telly.
It's really easy and addictive to slump in front of the box and difficult to get off the sofa and do things if there's an excuse to watch X,Y,or Z. It's really easy to eat without noticing what you're eating when you're watching the TV and very easy to snack on stuff without really registering what you're doing.
Parking the kid/s in front of the telly is an easy way to pacify them and while that's fair enough if you're ill/need to feed a baby or something, it becomes all too easy to rely on the TV to supply entertainment, rather than take the children out to the park or something that involves moving around.
Exercise per se doesn't make people thinner, btw. It actually makes them more hungry and more inclined to consume more calories. BUT those who do have the will power to exercise regularly probably also have the will power to resist eating too many of the wrong things. Plus, they're doing things that stop them from aimlessly munching simply to pass the time.
Also, exercising and feeling fit fosters feelings of higher self-esteem - a lot of women suffer from low self-esteem or boredom IMO and eat too much junk/drink too much booze to compensate. We're all supposed to be beautiful and perfect, aren't we? But most of us don't feel remotely beautiful and very far from perfect. We feel inadequate.
Also, the over-availability of cheap, over-processed foods which become part of a vicious cycle, because the more of them we eat, the more we crave.
Plus a lot of the other things mentioned above.