Some observations ...
1. The USA was competitive in the middle distances through the 60's and 70's, then it seems we dropped off for the next two decades after the Moscow boycott.
2. Per the above observation, there is also the Pre effect. The death of Prefontaine certainly ushered in a new era of inspired runners, but by emulating Pre's training and racing philosophies they neglected the ability to train and race smart, a key for success in the middle distances.
3. The few Americans who had some success in middle distances between 1980 and 2000 - people like Steve Scott, Craig Masback and Sydnee Maree - weren't exactly the charismatic types that could draw talented youngsters to the sport.
4. The running boom of the 80's seemed to have planted the seeds of low-intensity/high-mileage training as the "norm", a training technique not necessarily in synch with a high-intensity/speed oriented approach to middle distance running.
5. There's also the transition from cinder tracks to rubberized tracks, coupled with more training on pavement and less training on natural surfaces, that may have hindered US middle distance talent relative to the runners coming out of Keyna, Ethiopia, etc. IMHO training on natural surfaces enhances natural form and minimizes the occurance of injury, while conversely the constant pounding on unforgiving surfaces would seem to increase the injury rate.
6. Eventually you'd think the wealth of the US would result in a more scientific approach to training, and that seems to be the case now. Things like the underwater treadmills and altitude masks were unheard of in the 90's, yet it is this new (albeit costly) technology that has allowed the US runners to overcome the disadvantages that came about per #5 above. Thank you, NOP!!