Most of what has been said can all be true to an extent, but sometimes the simplest explanation works best. Most of the top U.S. marathoners train to run 5 to 5:10 minute miles in the marathon and that's the pace they try to stick to and that's pretty much where they end up. That's a safe approach. Safe on many fronts.
1. You are less likely to get injured by potentially training above your capability.
2. You are less likely to blow up in a race by potentially competing above your capability.
3. *You are less likely to miss out on a payday by not being among the top Americans.
*For a lot African athletes, you are spending months away from your family and it's a matter of making the podium or else. Else could actually mean being in debt since your agent is fronting all of your expenses. You have no choice but to run fast. A lot of times they blow up, but that's the risk they have to take because finishing 7th or 8th for example and running 2:15 means almost nothing. Btw, sometimes the gamble pays off and they become the next marathon star.
Over the last decade, in WMMs, only Meb, Ryan Hall, Ritz and Rupp have actually raced with the intent of winning the race. In life, you get what you put into it. There is no way you are going to win a WMM and/or run 2:07 if you train to run 2:12 and race at that pace. If you train to run sub-2:10 and simply race to win as in run with the leaders for as long as you can, you have the possibility of running under 2:10. On the flip side, you also have a greater possibility of getting injured in training and/or blowing up in the race, but you never know. I use to think that the aforementioned athletes where simply more talented; the fact that they had faster PRs at shorter distances is what obviously made them better at the marathon. That's simply not true, there are a ton of world class marathoners who only have 28x PRs in the 10K. Just looking at U.S. marathoners like Bill Rodgers , Greg Meyer, Bob Kempainen and Ryan Hall for example, I don't think any of those athletes ran sub-28 in the 10K.
The other simple explanation is the NCAA. NCAA XC all-American's routinely do long runs at the equivalent of a 2:15 marathon pace. I am not saying they could hold that pace for a full marathon. but some of them probably could if they were training for a marathon as opposed to a 10K. In the U.S., most marathoner's don't start training for it until their mid to late 20s. That's not necessarily a bad thing especially if you are having success at shorter distances. However, if you can't achieve the USATF 28:30 10K standard, it only makes since to move up in events. Although the USA has a much larger population than Ethiopia and Kenya and probably leads the world in recreational marathoners, we have fewer true athletes training for the marathon. If your NCAA career is over and you consider yourself a long distance runner, immediately move up to the marathon.
A final note, we routinely refer to 2:15 marathoners as elite, perhaps they are USA elite, but the truth is, the marathon has progressed to the point where sub-2:10 is actually world class elite. You can't say you're world class elite when the winner of the race has done a victory lap, put on their sweats and is conducting interviews when you cross the finish line. If sub-2:10 is world class elite, that means the USA only 2-3 athletes in that category. In the entire IAAF table of events, that would make the marathon our worse event. Sad, but true! This is a crazy thought when consider our success in other disciplines and consider the fact the U.S. has over a 100,000 marathoners a year and half of the WMMs are on our soil.