I point to a number of factors (in no order):
1. Rise of video games and cable TV: gives kids something totally engaging to do, that does not promote fitness. Growing up in the 70s not a lot of my friends had cable.
2. African dominance. North American kids don't have identifiable role models. This is not meant to be racist, but I think it is fair to say that the perception is that one in 500 million will be competitive on the world stage, so why bother. Totally different in the 60s and 70s.
3. Availability of fast food/junk food. Again, in the 70s McDs was just catching on. The concept of drive-through and eating high calorie crap because we are all so pressed for time was not so ingrained (see points below).
4. A families' economic model has changed. In the 70s it was totally unnecessary to have both spouses work because so few did. As women began to become equal earners, single earner families fell behind - thus pressuring them to have both spouses working. This created a situation were men either had to earn more, so as to keep up with double income families, or their wives had to go to work too. As men needed to earn more, being a part-time employed dirt bag runner became less feasible. As well, as workers, or prospective workers, looked to the future, they started to get more education so as to earn more. Education became more important as did getting an early start on one's high earning career. This also made parents realise that to keep their children competitive in getting the best education and eventually the best incomes, they had to make sure their kids were doing all sorts of resume building activities (likely that take away from having the time to run 80 mile weeks). The result of all of this is that whereas in the 60s one could finish high school, screw around for years and then in one's late 20s jump into a career that was not far behind everyone else's, these days one had better go to university, get 2 degrees and start one's career right away, or risk being way behind every one else that did that (which is way more common now). [Note to the old times, I am not trying to say that having a career in the 60s or 70s was not hard, just that there is a huge focus on education and qualification and expertise that I don't believe existed back then.]
5. Urbanization. More people than ever are living in big cities. They are more expensive than small towns. Accordingly, people have to work more and play less. As well, big cities offer less opportunity for fitness, lots of opportunity for eating crap, and few jobs that require physical labour.
So why are we having a resurgance of sorts? I would guess a combination of demographics (I have no idea, but since the other factors haven't really changed it has to be something) and it took a while to realise the dangers of the above points. After a generation of latchkey kids sat in front of the TV eating poptarts while their parents went out to be super moms and dads, people realized the dangers and started trying to get back to where we were.