The dyes that were used were VERY expensive and therefore, only royalty or those of wealth could afford them.

Because the dye for purple used to come from Sea Urchins and other Mollusks and it was very difficult to obtain and required vast amounts of harvested Urchins that required hours of processing to get the purple dye. Therefore it was VERY expensive to produce - so much so that only royalty and the very wealthy could afford it. From WikiPedia: "The actual color of Tyrian purple, the original color purple from which the name purple is derived, is the color of a dye made from a mollusk that in classical antiquity became a symbol of royalty because only the very wealthy could afford it. Therefore, Tyrian purple is also called imperial purple. Tyrian purple may have been discovered as early as the time of the Minoan civilization. Alexander the Great (when giving imperial audiences as the emperor of the Macedonian Empire), the emperors of the Seleucid Empire, and the kings of Ptolemaic Egypt wore Tyrian purple. The imperial robes of Roman emperors were Tyrian purple trimmed in metallic gold thread. The badge of office of a Roman Senator was a stripe of Tyrian purple on their white toga.[10] Tyrian purple was continued in use by the emperors of the Eastern Roman Empire until its final collapse in 1453. Royal purple: Medieval Europe This shade of purple is bluer than the ancient Tyrian purple. In medieval Europe, blue dyes were rare and expensive,[11] so only the most wealthy or the aristocracy could afford to wear them. (The working class wore mainly green and brown.) Because of this (and also because Tyrian purple had gone out of use in western Europe after the collapse of the Western Roman Empire in AD 476), Europeans' idea of purple shifted towards this more bluish purple known as royal purple because of its similarity to the royal blue worn by the aristocracy. This was the shade of purple worn by kings in medieval Europe."

Purple dye used to be very expensive. Royalty could afford it. http://pffc-online.com/mag/paper_history_shellfish_royalty/

This answer is more religious however in the devine mercy our lord is shown with both red and blue rays from the sacred hear,the red represents life of souls the blue the souls righteous,combine these two colors you get purple. Which jesus wore at the crucifiction .The color is worn by noble people and kings and the incription on the cross read king of the jews. so in a way it's a color of redemption for anyone who wants to believe..

'Cos no-one else wants to wear them!

mystery and socerery

I don't really know but I'm guessing that maybe a "royal" long ago developed a fondness for the color and decreed the wearing of it exclusive to those of royal blood only and thus elevated it to a certain status.

Because the poor hates that color. Hence Robin Hood perfered "green" over dirty purple. There's a reason why there are so many assasinations - The Royalty tries to stand out.

the catholic church.