See here: http://en.wikipedia.org/wiki/Lippmann_plate
I should have been more specific. Lippmann plates cannot be copied and very few of them were made. They are thus not abundant as museum pieces, and due to the toxicity of Mercury, few are being made today. I am referring to the color plates only.
In addition, the very limited viewing angle makes them less than ideal as display pieces.
That is my take on why you rarely see them.