Purple, traditionally, represents royalty. This goes back to the Tabernacle times at least. So it's a pretty ancient symbolism. Also, because it was expensive to make, as has been said. A very prized and noble color. Also a beautiful one.

Why is purple a common color in Cadbury Egg Chocolate?

Because it comes around Easter time, when we celebrate the resurrection of Jesus Christ, King of Kings and Lord of Lords! Glory! Hallellujah! Jesus, Prince of Peace!