Typically when a new console comes out, there's a good chance the main games for that console will make the transition, mostly due to technological advances, which is the initial reason why the console was made.
Pokemon X and Y is definitely a step up, quite a big one in this franchise. Even though I don't think it looks that impressive, it seems like this was only possible on a handheld with sufficient power - which the 3DS obviously has. Of course they could make a downgraded version for the previous console - Call of Duty and other major franchises have done this before - but from a marketing perspective, I don't think it would be that great, sales wise.