In the US Army why does the junior enlisted stand to the left of the NCO?

Answer 1

In the days of the Civil War, swords were carried on the left and drawn with the dominant right hand. Higher ranking soldiers were more skilled so standing on the senior's left would not interfere with sword handling.

Answer 2

The right hand is reserved for an equal or trusted counselor. This is tradition as old as civilization. To the left and one step behind is a visual respresentation of deference to a greater authority.

On a personal note, one of the funniest things I have ever seen was watching a Lieutenant General and his cadre walking across a flight line. It was like watching a flock of geese. They were in perfect step with the LG, ranged in ranks from a Brigadier to a second lieutenant. Each marched on behind the higher rank, one step behind, one step to the left. the wedge of them floated across the flight line at Tinker AFB in OKC. Each of the lower ranking officers was clearly impressed with him/herself, being fortunate enough to be permitted as part of the general's party.

A Sgt. Major was in their group, but he wisely took a different route and met them at their destination (he took a Jeep and drove around to meet them when they arrived). As did I with the LG's car.