This is mainly to do with how hard it is to make purple dyes.
It's hard to make purple dye because the first real source for the color was snail mucus.
Mythology tells us that Heracles was walking on the beach when his dog found one of these snails and began to chew on it. The dog's mouth filled with the purple saliva. This color became what is today called Tyrian purple.
The snails were bolinus brandarus. The spiny dye-murex. The snail has a mucus that it uses to paralyze its prey. It is also a defense mechanism. To get the stuff for dye you either crush the snail or you milk it. You milk a snail by poking at it until it secretes the mucus.
Then you make robes for your king or queen.
The dog you made us get runs off down the beach
despite being called over over over he refuses to come back
We got him because of his sad face
because of the two small scars across the front of his nose
Little square bites of gray in the blonde
I say you made us get because you made us go
to the shelter to the hill the shelter sits on
I like the dog fine I wish he’d come when you called him
The beach has tuned itself into a throb of seaweed
after a storm the sand is raw and untouched
Over the small hill the dog is tearing at the earth
shaking itself in a tug of war
He has eaten a spiked shell has lodged it in his mouth
A hundred arrow-sharp spines piercing its mouth
now stained purple the eyes filled with purple saliva flooding purple
All around him horseshoe crabs line themselves
Their heads inland and scorpion tails out to sea
each and every one of them dead
They wanted something somehow in their blind eyes they knew
it was just over this or that scrub of bleaching grass
They died crawling from one world into another