I think it's simple: US Marathon training has been lacking the specificity in training needed - it's what Canova has been preaching that last 8-10 week cycle of marathon specific work where Marathon pace long runs are key to 30-35km.
I just found a log from 2001, where I was trained by one of America's top marathoning minds (I was a 14-min guy) and my workouts consisted of 6-8 x 400m at 65 sec, 6 x 800m in 2:20, and 4-5 mile tempo runs at 5:30 pace. Looking back, this training doesn't make sense to run sub-2:18. Averaged 75-110 mi/week.
I wish I could have my prime back, where I could hammer 20 miles at 5:10/mi pace and that would consist of training.