Too many Americans are moving up to the marathon too quickly, at too young of an age. There's no way they are going to be able to have long careers when they are racing marathons at 24 years and younger (plus the training mileage necessary to be competitive). Too many talented young Americans are rushing their development and it's destroying them.
Everyone figures that they don't have enough speed to race on the track so they just move way up in distance. Instead of training speed (like Galen Rupp), they just concede and move up to the marathon. You can gain speed. It's trainable. If it wasn't trainable, then why do sprinters even practice?
Marathons are ruining the general population too. Everyone with a 28 minute 5k PR wants to put a 26.2 bumper sticker on their car. They are just going to destroy their bodies. Nobody should race a marathon until they can break 20 min for the 5k.