Here's a thought: what if Diancie is more than just a fun extra. What if it has a connection to the storyline?
Diancie is one of two Pokemon confirmed to be a stronger iteration of a normal Pokemon, the other being Genesect. Specifically, of Carbink, which Dianice is referred to a "mutation" of. Now, notice something about the stats between Carbink and Diancie? Diancie has a 100 increase in BST compared to the original, Carbink. But their base HP aren't any different. Hmmm...what increases a Pokemon's power by 100 BST, while keeping their HP constant? That's right, Mega Evolution!
Diancie is almost a Mega Evolution of sorts to Carbink. However it differs in not being a Carbink who's power was enhanced by some item, but by being mutated. And I believe it deals with the origin of Mega Evolution, AZ's Ultimate Weapon. AZ's weapon had the power to resurrect Floette, very nearly as strong as its evolution, while making it unable to breed and learn moves it couldn't naturally learn. Who knows what else this weapon could alter
I believe that there were Carbink close to, or maybe even being used as part of the Ultimate Weapon. The ones that survived were mutated into Diancie, with the same energy that created the Mega Stones, only it became a permanent part of their bodies by fundamentally altering their DNA. As such, there is a finite amount of Diancie, explaining their extreme rarity. Like Floette, the weapon rendered them sterile. And, as Carbink live for millions of years, they would easily still be alive after a mere 3000
This does mean that there could be a smaller, new group of Diancie created by the Ultimate Weapon. And it may not be the only Pokemon mutated by the weapon. Remember how Deoxys was a virus that became a Pokemon via a laser? What if AZ's weapon grazed a meteor?
