I agree with Mr. Doritos, but I'll address several points as well. Time is far too scarce today. A guy working "full time" is very likely to be putting in 60+ hours per week, not 40. And that's just his job; it says nothing of additional hours for social life (including kids) -- so most people will either have to sacrifice time for fitness or sacrifice sleep to get good at or develop a passion for running, either choice being a recipe for making it more difficult to see fast times from 'regular joes' today.
Getting down to the nitty-gritty, not many people can go sub-2:30 in a marathon with less than 90+ miles per week, let alone less than 70. It's pretty damn difficult to hit those numbers without doubles, and most people just don't have time for doubles these days.
In general, you have more sedentary jobs today, so there's tons of lost fitness there. I mean seriously, if your resting metabolic rate increases significantly just from standing as opposed to sitting, how much harder is it for the sedentary guy to hit the same fitness goals as the guy with an 'active' job (e.g. package-delivery boy, line-cook, whatever). Even within big corporations, 'professionals' don't have to walk as much -- You can communicate so much more easily by sitting at your computer or in front of your phone or other smart device. So there's no reason to walk to a meeting in a different room, you just video conference. You don't have to physically deliver the specs to the other engineers, you just email them, etc., etc.
Diet also plays a huge role. The average restaurant meal has increased in portion-size. The average snack is going to have more sugar, or sugar more sneakily hidden. You have a greater prevalence foods with artificial or processed ingredients today, unless you actively go outside that and hop on a movement of some sort (natural, organic, non-GMO, vegan, or whatever your fancy), which is economically prohibitive for many 'average joes.'
So when you add all these things up, the hill that one has to climb to get to a good level of fitness is steeper than it was in the '70's, and that makes it more difficult to breed passion as well.
The "it takes a village" and "team" movements of the 1990's and all years since really made it harder for the running boom to return as well. Running / track & field is an individual sport, but how many care about that when we are constantly reminded that we 'need' lots of people helping us, or that we need to work as a team, work on our team spirit, be a part of a 'community,' be 'social-'ly connected, and to 'share' ??? The pride of individual accomplishment has been so diminished that it's hard to re-birth a movement that relies on an individual pushing himself. So only team-sports get much thought.
And the media isn't helping. While the national championships in track and field were happening, my local news stations' top sports story was a high school lacrosse match. Other b*s* made it stories #2, 3, and 4, and track was never mentioned. During the 2013 World's, I remember the sportscaster remarking that it was a "slow" sports day. WTF?!