A lot of good points have been made, but as a person who started in the sport in the early 70s and is still involved today...I've witnessed the evolution. The 3 big factors are:
Our bodies have changed, a lot of girls people today call skinny, look like a typical female when I was young. Our diet and sedentary lifestyle is not conducive to endurance sports. When I first took the Presidential Physical Fitness test, most of the kids passed it, in talking to PE teachers in the mid-90s, only a hand full of students meet the standard. In fact, a lot of runners where discovered from that test. I don't think kids take that test anymore, which tells you a lot.
Although T&F in general was more popular a couple of decades ago, it was still the Olympics which created the icons/sports heroes. The 1980 Olympic boycott did an immeasurable amount of damage. That 1980 team would have been a powerful team and would have produced lot of stars…kids are most impressionable in terms of sports at about age 12-16; 4-years means we lost a generation. This combined with the African dominance starting in the late 80s caused the 90s dark ages.
Rise in the popularity of soccer, lacrosse and competitive cheerleading, etc., siphoned off some of our best athletes.