Probably a combo of things.
- Training improves. US distance running in the 90s was in a terrible state as competition (Africans) got way better and Americans didn't. The US training system I think wasn't particularly good, lots of hard anaerobic workouts that for too long during the season and not enough endurance building. Africans came to dominate mid-d and distance running with huge aerobic bases. I think since they American training has switched to develop more endurance and therefore much faster runners.
- Belief. Some barriers are mental. Before Bannister people illogically thought someone would die if they broke a 4 minute mile. A decade later HSers were doing it. By 2000 people thought Americans could no longer compete against Africans, then Webb comes along with a 3:53 in HS (even on that lots of hard anaerobic work and less base building training..I should know, he was a teammate of mine) and shows we can compete. Kids coming up starting to run in the years after that saw the legendary Webb and realized they can start running faster than kids had been running before that.
- Obviously shoes. Times across the board have improved a ton since late last decade when super shoes and super spikes started being made. We'd be seeing much fewer crazy performances, and the top performances would not be nearly as good, from HS to pro running, without super shoes.
- Bouncier tracks for those that have been replaced lately add even a little bit more on top of the shoes.
- I guess bicarb also helps for those who take it.
- Also I think probably pro runners are marrying more. Running became a professional sport just like two generations ago, so I'd say in the past 20-30 years more really fast men and women starting marrying each other and passing on a desire to run to their kids. This of course only applies in certain situations, but we know of course of some examples of top runners these days being the kids of two pro runners.
Mostly it's better training combined with the belief that anyone can compete with Africans, that have allowed non-Africans to catch up to Africans, and then the super shoes have improved performances significantly across the board, which is why we are seeing crazy times, and many more people running what used to be top times, across the board at all levels.