A lot of people, though certainly not all of them, were running for performance rather than for health.
This is a good summation of what happened. In the mid 70's we were all running 75 + mpw (even our JV guys) in HS. We all assumed, like the general populace that running a lot wasn't necessarily good for you, it was what you did to get faster.
Nobody had a computer or connectivity, Pong wasn’t enough to create an addiction, we didn't have a girls team (not sure if this makes you slower), or a soccer team, the aerobics craze was a decade away, jogging had yet to catch on, Athletic Attic was the only real running store, shorts were still short, coaches could cuss you out and publicly humiliate you without fear of losing their job...these, and more, all conspired to create a more conducive environment for fast times.
The other thing that helped was that running was so far outside the mainstream that it took a "special" type of person to become obsessed with running. That specialness has been diluted and probably lost to video games.