Running was never more popular than football. But football wasn't as popular then as it is today.
It did help that in the 70s/80s there were American stars winning big events, like Shorter, Rodgers, Salazar, etc.
Running also fits well into a Gen X mindset. Independent, latch-key kids who had to find their own way and were willing to work their tails off. Largely ignored by the adult world and driven by instrinsic goals. For Gen Xers quantity and quality were the same thing. Many of them didn't train very smart, but they did train hard.
Running doesn't fit as well into the Gen Y mindset, which is one of having life hyper-organized by adults and also one of cooperative learning and "teams." Gen Y is just much better suited to team sports like soccer. They work great in teams and work hard, but more for extrinsic reasons. And, they seem to take losing in stride much more than Gen X ever did.
What do you mean by Gen X and Gen Y? Because I've heard born in 1964-81 or so for the former, and 1982-1998 for the latter. Seems to me Gen X was responsible for the crappy '90s when running an 8:59 3200 was enough to lead the nation many years, and Gen Y is responsible for the platinum age of high school running happening now. You may want to rethink your theory.