During a recent interview, Junichi Masuda and Shigeru Ohmori of Game Freak revealed that new Mega Evolutions won’t make it into Pokémon X and Y. Even though Pokémon Omega Ruby and Alpha Sapphire are both compatible with X and Y to a certain degree (no solid information at this point), the developers decided not to transfer the new Mega Evolutions to the previous installments.
I’m sure this piece of news is going to disappoint some X and Y fans, but hopefully Pokémon Omega Ruby and Alpha Sapphire will be able to make those people forget about that by being wonderful games on their own.
Source: Nintendo Life