Remake, dot you mean redesign? The animation system is a lot mor advanced than gen 5, where you just teetered left to right and your body compressed a bit when you attacked. They have unique animations for almost every move, therefor needed to re write the animation coding, therefore going through all 649 other Pokemon and re animating them just so they'd be transferable. Then of corse, they didn't have enough time to make 150 new Pokemon, so there was only 69.
Or of course,its just how GameFreak wanted it.
(Sorry if this wasn't the answer you wanted, the question wasn't that clear.)