There seems to be a lot of misapplication of pysiology when it comes to discussions about doping.
The potential benefit of epo comes from it's ability to raise hemoglobin levels, which could in turn increase oxygen delivery during exercise. Additionally, for this to help, the potential benefit must be outweighed by any adverse affects, such as increased blood viscosity.
Each athlete would presumably have an optimal hemoglobin level where that aspect of their performence would be best. If the hemoglobin is at that level by natural means (altitude, ect), then epo could not possibly help. If the hemoglobin is lower than optimal, than it would.
Additionally, it is possible that increasing oxygen delivery would not improve performance, as oxygen delivery is only one of many factors and may not be limiting in a particular athlete. In such a case, epo would again not be helpful.
I have no idea which category elite distance runners fall into, but there certainly is a reasonable physiologic explanation for a lack of benefit in top athletes.