2-3-4-4-2 Heh, guess for me the time just passed really fast from 5th gen.
It might've just seemed faster because of B2/W2. And I would imagine the fact that Ninty really needs to market the 3DS & get better sales could've had something to do with it.
In fact I wouldn't be surprised at all if they only add around 100 new Pokemon as the other 600 already have 3D models thanks to the Pokedex 3D Pro/PBR/etc. That'd leave them a bit of time to work on the other aspects, like the overworld and story/features & whatnot.