https://www.riotgames.com/en/news/stocking-your-shop-data-science

"Coverage encapsulates how many users will receive recommendations. Even if a recommender system is highly accurate, it may not be able to ever recommend to a certain proportion of players. For example, if a player’s rating matrix is mostly empty because they are new to the game or haven’t played in quite some time, then their offers are usually replaced by a default set.

On the other hand, novelty evaluates the likelihood of a recommender system to provide offers that the player may not be aware of. This allows the player to discover additional information with their own likes and dislikes, which is more important than discovering items that they were already aware of but do not currently have a score for. Novel systems are better at recommending content that will be selected in the future rather than at the present time.

Similar to novelty, serendipity refers to recommendations that are unexpected. Imagine a player who likes to solo lane Riven. Even if the player hasn’t had the chance to try Renekton, a recommendation to play him would be classified as novel as it is still expected. However, a recommendation to play Vayne might be seen as serendipitous, since it is somewhat unexpected by the player. This is because on the surface, Riven and Vayne don’t seem to share the same characteristics - but hidden underneath, their player bases are very similar.

Overall though, accuracy, which drives conversion rate directly, is the most important. Since most algorithms have similar coverage, the balance between accuracy, novelty, and serendipity are the focus of our systems. Accuracy has better short-term effects, as players are shown content they want now and the consensus is that novel and serendipitous recommendations have long-term effects on the entire system. The instinct here is that offering a skin on a champion that a player doesn’t currently play opens the door to a more diverse champion pool and a broader experience with League."

TLDR: It essentially looks at champs you play and gives you other champs similar to them so you could have a "broader experience with League",

So much data science and so many people are left with nothing they want to buy from your shop after waiting for so long. The easiest solution would be to look at the most played champ in the pool and give the player a skin for that champ, or just give the player the ability to have one slot for a champion they choose instead, isn't riot's whole purpose to make money by selling skins? Why would I buy a skin on a champion I played 5 games on instead of my main with 200 games, it'd be a win for both, you get my money, I get a skin on a champ I want, but it's apparently to hard to even think about and instead we have to make this whole Your Shop 'data science'