When Frank Shorter won the '72 Olympic Marathon, running suddenly became the thing to do, and it became more mainstream. A writer named Jim Fixx put out a book about running that became a best seller, and it introduced a lot of people to running. (I don't think the book said anything extraordinary, it was just the right book at the right time.) It was no longer considered weird to be out running. Those two events could be credited with starting the "running boom".

Also, as a previous poster stated, HS kids were less distracted with after school jobs.

The more runners you have, the tougher the competition, the faster the times.

Was running more popular than football? No. But it was considered less geeky to be a cross country runner than in the 60s.