>>49476293i think 2d worked good for pokemon with atypical posing or body shapes like birds with big wingspans and other long or wide monsters, except maybe gen5 due to sprites now being fully animated.
in 2d games the sprites are most always static and all fit into the same dimensions (56x56 in gen 1, 80x80 in gen 4, not sure about other games dimensions) which meant the pokemon only needed to be designed to look good in a single position within those dimensions. in gen 5 the full animation becomes a little bit of a problem since this gives them more dynamic motion and the need to keep things from looking warped and distorted in the 2.5D style. then 3d shows up and you have another axis to worry about while still confining all models to the same size box like in 2d games.
but what do i know, im mostly just talking out of my ass