It's because manufacturers have made #6 and smaller wires in colors for decades.What is the reasoning for not being allowed to re-identify grounded & grounding comductors #6awg & smaller? Just curious on this one. Is there something magical that happens when you jump to #4awg?
Also in a typical panel you only have the main feed that is 4awg or larger, or at most a handfull of other feeders. Whereas you may have dozens of smaller conductors that can much more easily become a confused rats nest, and if you lose a marking.... Plus tape marking is just easier to see on bigger conductors. Not sure if that was part of the original thinking but it makes sense to me.It's because manufacturers have made #6 and smaller wires in colors for decades.