>>15128517XY has been in development since HGSS. Because there were SO MANY new pokemon, they decided to have a gen in between. So most of the new pokemon were added in gen 5. That's why there were so goddamn many, compared to so few in gen 6.
So apart from all the new pokemon, gen 5 barely made any progress from gen 4, whereas 6 added and revamped so much shit.