Lots of causes - some are more difficult to fight than others.
Technology - little things such as cell phones letting you answer the phone on the couch rather than get up and walk to it. More captivating sedentary media with the web, games, TV, etc. sucking up our time.
Bad food - a lot more commercial junk food with hidden sugar and calories.  I'm always amazed to see the line of cars at the McDonalds drive-through when I go to work in the morning.
Easy to blame "weak-willed" people, and there's some truth to that, but the board is tilted against us.