History of Sub 2h 20m marathon times in the USA

I came across this graphic recently showing the number of sub 2h 20m marathon performances per year in the USA. Although it stops at 2012, it clearly shows the surge of fast times back in the late 70's and early 80's.

As for the reason? The first running boom back around 1980 was fueled by mostly young men who trained a lot. The current boom has a much older age profile and fewer young men are doing the high mileage that their predecessors did.