So I was reading this question and stumbled into part of an answer:
Actually, there were systems that used Green and Orange in the past, but they were used to expand the range of the basic CMYK print, but these days the pigments are so good that hexachrome system was closed.
I was a student when hexachrome came around and I've discussed it in class before now that I teach but, to be honest, I haven't really seen it in use (other than a magazine I had that introduced the idea of hexachrome). I've had this discussion with colleagues before and no one really knows what happened to this technology.
How have the pigments changed? I'm sort of having problems understanding which properties of a pigment can improve to the extent that the extra inks could simply not be used (green, orange, and I believe violet was sometimes also included).
Were the costs for acquiring such a printing press too prohibitive for the different it actually makes? Was there a lack of support coming from the software? Is hexachrome widely used but I just live in some sort of isolated bubble? :-)
ETA: Wikipedia says:
Hexachrome was discontinued by Pantone in 2008 when Adobe Systems stopped supporting the HexWare plugin software.
But a quick search for CMYKOG printers suggests the technology still exists...