It makes perfect sense. All Kalos Pokemon are completely new, so giving them mega evolutions would be a bit silly, as it is just adding another form to Pokemon we have never seen before. The main purpose of mega evolution is (as far as I can tell) to add something new to old favourites. Pokemon like Lucario, Garchomp or Mewtwo. Or add an "evolution" to Pokemon that are alright as they are, but could do with some spicing up, like Mawile or Absol.
Following this reasoning, there is really no need to add a mega-evolution to any Kalos Pokemon. I don't believe that any Unova Pokemon mega evolve either, and probably for the same reason, as those Pokemon are still fairly new.
Because of this I believe only Pokemon from Gens I - IV get mega evolutions. That is not to say later Pokemon won't mega evolve in future games, just not right now.
