Before Gen VI, we had 649 Pokémon. Which is a lot. There comes a point were it becomes inefficient to produce large quantities of Pokémon all at once. If you ran a small convenience store, and you still had a fair stock of bread, would you call for 50 more loafs or 150? You would most likely call for 50 loafs, since 150 would leave you overstocked.
Plus, there are only so many ways that you can alter the stat, type, and movepool spreads without it becoming either redundant or overpowered. I know that Game Freak still has a ways to go in terms of combinations, but it is a more strategic and productive move in the long run to take their time and save some of those combinations for later Gens. Furthermore, those 649 Pokémon from the first five Gens are still perfectly usable, so why ignore them in favor of unnecessary new Pokémon?
Even further is the fact that having such a large number of Pokémon isn't easy to maintain, as they have to create the models for each and every Pokémon variant in the game (including forms, gender differences, and most of all, Shinies, which alone double the number of models that they have to make), animate them in several different ways for different situations, add Pokédex entries for every one, etc., etc. It takes up a lot of data, and XY were already very full games with loads of other features.
And lastly, there is also the matter of having to come up with names for every new Pokémon while also ensuring that they aren't infringing on any existing trademarks.
(And this is, of course, the point at which mega evolutions come in, as they are a brilliant concept from a production standpoint since they allow Game Freak to circumvent a lot of these issues while still introducing new content.)