Kraig can correct me if I am wrong but as I understand it the military chose the fixed 10X as high enough magnification for long range shooting, with an excellent field of view, and without worry of mirage interfering in any potential operating environment (jungles, deserts, etc)
Fixed magnification allows a more compact/lighter scope with fewer moving parts that can potentially cause problems.
It is a compromise that people who are not snipers most likely do not need to make. A good variable will allow high magnification when mirage isn't bad, allowing more precision shots, but can be backed down if mirage is a problem or if more field of view is desired.
If there is a problem with the variable adjustment makes my scope unusable, I get irritated, and a P-dog gets to live another day. If a sniper has a problem with his variable adjustment, he is no longer an effective sniper.