It is a matter of cost versus benefit.
While recall animations and complex dances and emotes are really fun to watch in game, they consume time as they not only need to be animated, but ideated, conceptialized, sketched and studied beforehand, then surveyed by QA and other pertintent departments afterwards.
It is not easy to simply add them, and in the case of champion VU's, each time invested in an update pushes back the release date of not only the one that is worked on, but every other that will come after it. In the case of skins, there is just a delay in when the skin will come, but by adding a recall animation to the Miss Fortune update, you are relegating Ryze/Teemo/Fiora/whoever update by the time that is required to process the feature.
Add to the fact that, as has been mentioned before by Riot Games, their animation department is a natural bottleneck for them, as they use tradional animation for their models rather than motion-capture animation, so it is difficult for them to find enough experienced animation artists to support the workload (or at least was this way when they posted it).