Some guns work best with a high elbow, others not so at all.
I believe Jeff has stated the reason you're looking for. Take a look at the grip configuration over time. The Trapdoor Springfield, Krag and Springfield 03 all had straight grips which necessitated a high elbow to help wrap your thumb around the far side of the grip.
The C stocked Springfield with a pistol grip and subsequently the Garand and M14 allowed for a bit more relaxed elbow...and the M16's full pistol grip almost requires a low elbow position.
