As already said, because they can and because white is a popular colour now. By artificially elevating the price of Oryx White to a premium status, it makes the cheaper Pure White seem like a relative bargain (despite their flat white being free before). Of those who want a white car, some will go for Oryx, but most will go for Pure. Win-win for VW.
Personally, while I like the fact that Oryx is closer to Candy from a distance, I don't like the pearl and shimmer-effect up-close. OK, I haven't seen a full car dressed in it, but I've seen other makes with similar pearl white, and it gives it a bit of a feminine look if you ask me!