I don't slam American marathon, I simply analyse a situation which is very clear.
Forget, at the moment, Hall, Ritzenhein, Keflezighi and Abdirahman. Look at the situation behind them.
Last year, US had 16 athletes in top 500 in the World :
2:07:47 Dathan Ritzenhein (30.12.82) (9) Chicago 7 Oct
(69 in the World)
2:09:08 Meb Keflezighi (5.05.75) (1) Houston 14 Jan
(126 in the World)
2:09:30 Ryan Hall (13.10.82) (2) Houston 14 Jan
2:09:47 Abdi Abdirahman (1.01.77) (3) Houston 14 Jan
2:11:06 Brett Gotcher (1.09.84) (4) Houston 14 Jan
2:11:24 Andrew Carlson (27.04.82) (5) Houston 14 Jan
2:11:41 Mohamed Trafeh (1.05.85) (7) Fukuoka 2 Dec
2:11:45 Ryan Vail (19.03.86) (8) Fukuoka 2 Dec
2:11:53 Fernando Cabada (22.04.82) (7) Houston 14 Jan
2:11:56 Nick Arciniaga (30.06.83) (8) Houston 14 Jan
2:12:03 Jeff Egglestone (1.10.84) (16) Chicago 7 Oct
2:12:23 Jim Carney (24.03.78) (9) Houston 14 Jan
2:12:29 Jim Grabow (20.09.84) (10) Houston 14 Jan
2:13:41 Ricky Flynn (20.07.87) (12) Houston 14 Jan
2:13:42 Pat Rizzo (25.07.83) (13) Houston 14 Jan
2:13:50 Josh Cox (9.08.75) (14) Houston 14 Jan
How everybody can see, 12 among the best 16 times 2012 in US were during OT in Houston, that seems to be the only marathon really interesting American runners.
If we go to analyse what the above athletes did this season, we can see that 10 of them didn't run any marathon, for different reason (may be also somebody retired from competitions), and these are :
Dathan Ritzenhein (he will compete again in Chicago), Meb Keflezighi, Ryan Hall, Abdi Abdirahman (all these athletes are booked for a major marathon at the end of the season), Brett Gotcher, Ryan Vail, Jim Carney, Jim Grabow, Ricky Flynn and Jos Cox.
If we look at OG 2016 (but also WCh 2015 in Beijing), probably we have to cancel Keflezighi (41 for Rio), Abdirahman (39), Carney (38) and Cox (41), without forgetting also Hall and Ritzenhein will be 34 in the next Olympics (and in any case they will be interested in running WCh 2015).
Only 6 athletes in this group ran a marathon in this spring, and with very poor results :
Carlson 2:17:16 (7) Houston 13 Jan
Trafeh DNF Otsu 3 Mar
Arciniaga 2:17:05 (7) Los Angeles 17 Mar
Eggelstone 2:14:57 (17) Paris 7 Apr
Cabada 2:18:23 (16) Boston 15 Apr
Rizzo 2:16:05 (12) London 21 Apr
So, at the moment, in US there are only 2 athletes under 2:14:00 in the season :
Jason Hartmann 2:12:12 (4) Boston 15 Apr
Michael Reneau 2:13:53 (9) Duluth 21 Jun
It's clear this situation doesn't represent the potentiality of a Country as US.
This means there is something wrong in the approach to the marathon, as methodology, but also as strategy and interest.
When there is some athlete with the talent for becoming one of the best in the World, nobody gives him time to grow in the right way, since Marathon is something to LEARN before having the experience for being competitive in top championships or in some major.
For example, in my opinion for Dathan Ritzenhein was a big technical mistake to run NY as first marathon. I discussed with Brad about this fact, and he told me agreed with my opinion, but there were too many pressures by the Company for putting him there (and he ran 2:14, without learning anything about how to manage a top competition).
Secondly, when some athlete is near a jump of quality, he doesn't try to build his experience competing in some top marathon abroad, but continue to stay in US running or small marathons (where it's possible to win some prize), or big marathons without any chance against Kenyans and Ethiopians, losing immediately the motivation.
Third thing, the athletes running marathon usually don't have good times in shorter distances, and can't fill the gap of talent in this direction.
Forth thing, Americans go to marathon when already too old, so they have no time for developing their career at their best.
For all those reasons, their training is more "amateurish" than professional, lacking the right volume and right intensity for running fast.
For example, I don't understand because an athlete like Ryan Vail, still young and able running under 27:45, doesn't start a project for becoming marathon runner full time, since in my opinion has the talent (I don't know mental, but of sure physical) for running under 2:07 looking at Rio 2016.
May be I don't know very well the American market. May be many athletes can earn more money running shorter events, because the American circuit of Road Races provides very good prizes for US runners.
Anyway, I speak about the fact. It's up to who knows to explain the reasons.