Because
1. the old animations were run in-engine (as in, being processed and calculated in real time) as opposed to just playing a video. The running game scene for both the level up and the gameplay state itself is too much memory for low end devices. We seriously considered retrofitting the old ones too but that would have been millions of dollars and hundreds of hours of dev time.
Animating in-engine requires an art team that can do its work in Unity itself and takes up engineering and tech art resources that are custom to each champion. And you don’t know ahead of time whether the idea you have is going to blow memory budgets, so there’s a lot of iteration cycles to deliver a single level up.
Conversely, outsourcing a video to a vendor means we can do things in parallel and free up the gameplay engineers and artists to do other things. The video vendor makes the video and we plop it in when it’s ready. Vastly simpler on the production side AND engineering requirements.