Great question! One that bothered me so much I had to look it up a couple of years ago when it first came up to my mind. The answer I found then was staggering.

Before it was first synthesized in 1856 by Sir William Henry Perkin it was created from a certain species of sea snales from the Mediterranean sea. Reports say that it  took 10,000 of these snails to produce just a single gram of purple dye. For this reason, purple was worth more than its weight in gold and was never used in national flags because it would be too expensive to manufacture. 

The only people who could afford such luxury at the time, were royal families and they used the die to color clothes which separeted them from the pilgrims and served as a symbol of wealth.