I've got several hunches, beyond what's already been said here. I'll list them, then briefly discuss:
1) 10K was the default race distance, not 5K.
2) races were sponsored by running clubs, not charities giving out a trophy to every finisher and encouraging the wearing of pink ribbons and self-congratulation
3) the couch-to-5K model didn't exist
4) although women certainly competed, the scene was dominated by men
Discussion:
1) This may be hard to believe, but road 5Ks were rare back in the early 80s. I ran a few in Central Park, but mostly is was 10Ks and 5-milers. (5-milers were much more common, perhaps because that was a standard XC distance.) With 10K as the baseline, races naturally drew on people who were capable not just of completing a 10K, but of racing it. THERE WERE NO WALKERS.
2) The near-wholesale capture of the American road racing scene by charity races and other do-good groups has accompanied the serious erosion of running clubs. Charities aren't in the business of encouraging or rewarding great race times and the hard, optimal training required to notch them. They're in the business of milking every single entrant not just for "additional" gifts to the charity, but for actually BECOMING a fundraiser--a sort of pyramid scheme that has almost ruined, for example, the St. Jude Marathon in Memphis. It's all about fundraising. When you pull focus away from training, of course (average) race times slow.
3) The couch-to-5K model aims to take out-of-shape, non-athletic people and get them to the point where they can finish a 5K. I didn't say "race a 5K," because that's not the point. Again: when the focus is on using running to lose weight, make you feel better about what you see in the mirror, etc., focus is naturally being pulled away from racing the distance and the sort of training--and training tips, and conversation--about all that. I'll never forget the day at the Tallahatchie (Bank of New Albany) 5K in New Albany, MS when they waited to announce the winner until every single person in every age category, young to old, male and female, had been duly announced, walked up to get a trophy, etc. By the time the winner was announced and went up to collect his big trophy and check, almost half the crowd has disappeared. I realized then that the erosion of American distance running--and distance running CULTURE--was even more advanced than I'd believed possible. How can you make the winner wait through half an hour of awards to people who DIDN'T win the race? How can you not celebrate excellence--true excellence, as in "the guy who won the thing"--first? So it goes.
4) The feminist in me hates to say this, but American distance running culture has been feminized. It's been feminized to precisely the degree that 1) every finisher gets a medal (that's a mommy reflex: make every kid feel special); and 2) charities such as the Susan G. Komen Foundation, ASPCA affiliates, and similar do-good groups dominate the local road race scene: charities that, while doing terrific work for their causes, inevitably steal focus from, for example, the optimal training required to race the distance well. It's all about having "survived"--not survived the RACE, but merely making it to the starting line alive rather than succumbing to cancer, rape, or badly bruised feelings.
