Probably because the only members of the initial cast that had knock-backs were Alistar, Anivia, Janna, and Rammus, and two of those have very small knockbacks. Adding animations at that point in time was relatively meaningless, as only Alistar and Janna could actually displace people a considerable distance, and that was a lot of animation work for just two characters.
As League grew, I suspect other things always took a priority over creating a specific animation for a niche case. Even now we really only have a small number of characters with a knock-back sufficient to require this animation. This leads to an awkward situation where designing a knock-back animation for this many champions is a LOT of work that takes animators and artists away from more important additions and changes, so it probably always gets put on the back burner.