The reason for the high elbow was because you were supposed to do most of the "lifting" with your right arm. The left arm only provided minor guidance. In fact, people used to dry fire holding the rifle in the right hand and shoulder only (everything here assumes right handed shooters). At the time, the left hand was not so good for support anyway, because NRA High power rules did not allow the left arm to touch or be supported by the chest.
What changed?
1. This position is not good with rifles having pistol grips, like the AR15.
2. NRA rules allow the left arm to contact the chest.
This is why the high elbow is not used so much today.
