This is something that's always irked me some, but - much as I love Diancie's mega appearance - wasn't it mentioned somewhere in the story that only Pokemon who were NOT native to Kalos could go into Mega form, for some reason? Diancie is a Carbink, and Carbink being native to Kalos should by definition not allow it, nor Diancie by extension, to have a mega form.
That was never stated, no, and if it were, then Pokémon such as Salamence and Abomasnow, which in the flashback to the AZ-era war were shown to be present in Kalos 3,000 years ago and can still be found there in the modern day, would not be able to Mega Evolve either since they are most likely native to Kalos as well as to Hoenn and Sinnoh, respectively.
Basically, a Pokémon being introduced in any given Generation does not necessarily mean that that Pokémon is native to that Generation's region.
It's like Sudowoodo. You see a grand total of one Sudowoodo in Johto, but they can be found in abundance in Sinnoh and Kalos, which could suggest that they are actually native to those regions.
You may be thinking of this quote from an online interview:
"X and Y will include all Pokemon from all previous games, offering the largest selection of Pokemon in any of the series' titles. But only specific Pokemon have Mega Evolutions; older Pokemon — like Mewtwo, Lucario and the Generation One starters — will have Mega Evolutions, but Pokemon newly introduced in X and Y will not."
http://www.polygon.com/2013/9/19/4744438/pokemon-x-and-y-switches-up-the-classic-story-formula-to-focus-on
... but that is simply referring to the mechanically-oriented, out-of-universe logic behind the distribution of Mega Evolutions. It has no bearing on the in-universe story logic, and additionally, the introduction of Mega Diancie does not contradict this statement, as all that it claims is that XY will not introduce Mega Evolutions for any Generation VI Pokémon. Mega Diancie was, of course, introduced in ORAS, not XY.