I feel it is a combination of the internet making the world small. Before the internet, unless you were world class, your frame of reference was your state, or less.
Now HS kids are "competing" against times posted nationally, it has raised the bar.
The internet has also helped to raise interest in the sport, share training methods, etc.
And yes, Alan Webb. He came along right as DyeStat and Mensracing were gaining steam. It was a one two punch that demonstrated to HS kids that we could compete at any level.
You look at where we were at, which was not much, then comes:
Webb, Ritz, Hall, Tegenkamp, Solinksy, Symmonds, and the flood gates have opened wide.
That group holds, or held, many AR's and medals from big time competitions.
It probably also helped a lot to have one of the greatest MD runners of all time, Lagat, join the ranks and set a new standard.
Remember when Stember, Jennnings, and the Hausers were big shit. Hausers ran 13:20ish, Stember ran 1:46 I think and cleaned house.