You know what, there’s kind of a missed opportunity with Diancie. Think about it, Diancie is, canonically, a mutated Carbink, yet Carbink is not related to how it’s obtained at all. I mean, how hard would it have been to have an event-distributed item that somehow evolves Carbink into Diancie? We have a Mythical Pokemon that hatches from an egg for pete’s sake, why not have a Mythological Pokemon that evolves from a regular Pokemon?
I can’t speak for Game Freak’s decision-making process, but that direction is obvious enough and uncomplicated enough that I am inclined to think they probably have some reason for not taking it – most likely because Diancie is portrayed (e.g. in the movie in which she stars) as a special Carbink, with powers to which other Carbink cannot aspire. You might liken it to the rarity of a perfect natural diamond – you can cut and polish diamonds to make them nicer, but ultimately there are some flaws and inclusions that will be impossible to remove, and Diancie needs to be a perfect diamond.