You know how most people love new toys and new things, right? {:3}
That is my point for not releasing the other 70 -- To keep the original XY game players feel fresh and give them some kind of new experience as in oppose to revealing all of the available new Pokemon like usual. (Note: Previous event exclusive Pokemon are not included, when I said 'revealing all of the available new Pokemon').
On the other hand that creates a 'reason' for the original XY players to buy the sister games, as the other 70 are only exclusively available for the sister game(S). And that will secure income for both GF and Nintendo. {XD}
Yes, yes, yes! I know my points weak and funny...:P Just sharing my thoughts. :3c