So either we pause adding new Pokémon until all existing have those new animations (2-5 each, for all ~450 Pokémon currently in), which means considerable time waiting for demanded species. Or we keep doing both side by side, then we'd have maybe 2 new Pokémon per major update and a random assortment of those with new animations and those without. Or - the option the team chose - we focus on new Pokémon, and thus new content, first, so we finish up all gameplay features before we start on a huge solely esthetic aspect.