Meet one of my favorite VW colors, Mountain Green.
What it comes down to is people buy cars to drive for 2-5 years, then sell them or return their lease. Why would they buy a colour that would negatively affect the resale value/options. Also, most people just don't want attention drawn to themselves or have that outgoing of a personality that a bright yellow or green statement to the world is right for them.
Both these statemetns came up repeatedly on another forum when discussing orange cars (the OP of a thread there had bought a bright orange new M3) and the general consensus from most posters was that it was too flashy, that they didn't like being the center of attention and that they thought resale would tank. Even the people that liked the color woudl never own a car in orange, yellow, bright green or red for those reasons.
I love cars, but the problem is they are like schroedinger's hobby. They're always in a quantum superstate of being both awesome and a huge waste of time and money... until observation momentarily forces them into one state or another.