To answer the OP's question- The runners that don't make it are either physically or mentally burned out. The physical element obviously relates to the intensity and volume they've put in as teenagers. The mental burnout side is no different than you see with kids who participate in year round travel programs in other sports. By the time they reach HS/college many of them have lost the desire they once had.
If you run for a perennial powerhouse HS program you're more than likely pushing it to the extreme. To keep improving you're going have to increase the workload even further to compete at the next level. I would argue that the success kids achieve in HS is something they'll cheerish forever. I've seen many kids succeed at the collegiate level from such programs.
The talented kids coming from less established or perhaps less intense HS programs obviously will have a upside when they reach next level if they have the desire to be great and are put in an environment to succeed.
Cheers!