>>14321659k, i can explain why they didn't do this.
Before, When they made heart gold and soul silver, they didn't have to make new Pokemon. so instead they spent that time making overworld sprites. Now, making a sprite for every single Pokemon is hard they do that in the menus already. making 3 different ones depending on the angles it's standing at cause its following you? takes 3 times as long.
Now they had to make 3D models of Pokemon.
making a 3D model that can follow you around in the over world world would be way harder than sprites. and sprites take forever.
so don't expect to see it again ever. i'm surprised we saw it at all in the first place.