So, is there a technical reason why we can't dye certain armor?
There is. The skins have to be designed with masks and materials with instructions ("scripts" for materials to make them dynamic) to allow them to be changed to arbitrary colors.
Gear that can't be dyed wasn't designed with dyeing in mind, either purposely or as an oversight.

I think it was a design decision to make only crafted (craftable) items customizable, because crafting itself is meant to provide customization.