Concerning Udyr and Sona's Models

For what reason can't Udyr and Sona have their skins and animations tied onto their newer, smoother, models/riggings, Spirit Guardian and DJ? Skins like Odyssey Sona would be so much better if they had better animations, animations of which are_ already there_, just on another skin. The same goes for Final Boss Veigar and other skins that are a total upgrade from their base.