Too many characters, many of whom are too important, that in the lore all have nothing to do with each other.
Trying to write a singular narrative of League is like if you're playing DnD where there are 150 players and they play range from a dignified soldier to the literal god and each of doing their own thing and most of them have and will never meet each other. How would a DM make a story out of this? They can't; not even J R R Tolkien can make sense of that mess.
The only way to do about this is to keep the story small and animate all the short stories one by one. And here's another problem. While there are longer short stories, but most of the short stories barely fit enough screen time for 5 minutes of video content; which I guess can be fixed if they simply expand onto the materials more so it's not that big of a concern. The more critical concern is as previously mentioned, most of the characters in League are too disconnected from one another (despite the world of Runeterra is actually not that big, most of the characters don't travel, and have little with one another even with the ones living in the very same city while all being some kind of big-shot in their field); this will create a VERY disjointed experience and I doubt most people would enjoy it since they're probably here to watch different champs interact with each other despite the fact most of them actually don't in their stories.
They'd have better luck animating the skin universes, since those actually have more follow-able narratives, and character interactions aren't like a myth and actually happen on a far more regular basis. But most of them are knockoffs of something else, and are designed that way given skin universes don't really exist for more reasons other than to bump sales and give skins in the same skin-line slightly more meaning. I doubt it will make for compelling materials, but I guess they can still be enjoyable for the fans. Though a bigger concern is that obviously, Riot doesn't want to put skin universes over the main universe when trying new media.
In the end of the day, main universe League's world has MANY design and narrative issues that held it back making it nigh impossible to translate into any narrative based media. If they really want to do that, including but not limited to, just animating a series of short stories which is the most feasible route as far as I can see, they still need to put in a lot of work to make it so that champs actually live in the same world unlike how the vast majority of them live in their own secluded and isolated universe as they do in the actual lore, so that the animated shorts would actually be watchable and not merely a series of champion launch introduction video or something.
And even if they could do that, there's still the issues concerning the worldbuilding issues (too many high stake threats, too insane of presence disparity between characters, etc) that can also pose serious problems when trying to adapt them into narrative based media.
So yeah.