Why doesn't league have a game based around it's lore already?
One big reaction you see from a majority of players that are exposed to the game is that the trailers and the cinematics don't actually reflect the game. A lot of people are attracted to the cinematics and the universe but don't actually get involved with the game because the gameplay is not what the trailers attracted them to.
Whether it be an mmo rpg in the league universe or a story driven campaign that has a different story based on what champion you choose, I feel like it is a huge missed opportunity. The only thing I can guess is that having an actual built universe we can see take away from the imagination of the people reading the lore kind of like how people say that how books are better than the movie adaptation or "I imagined this better in my head" than what the writers actually created. They don't want to disappoint lore enthusiasts. The final thought that I have is that this is either a HUGE top secret project or they are saving this lore based game for when the game starts losing players.
A couple months ago before TFT, if riot said "we are working on a new project" and left out the part about it not being an mmo or lore based game I would have totally thought that they would have created a game based off their lore. People like TFT but I would MUCH rather have a lore based mmo or story driven campaign molded around the champion you choose than heavily rng TFT.
If you created an mmo per se, it would attract players from different game genres that not necessarily play mobas. People who play WOW might give the game a try and if the game is better than other mobas it would draw players to your game rather than the others. I know for a FACT league has a better universe that more than half the mobas out there.