Making a movie would be near impossible even with Riot's peak earnings. An average animated movie these days cost around 150-200 million dollars, not accounting for a marketing campaign; that amount of money would be immensely risky at this point in time, since characters in League severely lack the depth needed to carry a movie and the lore retcon means they were changed before much of anything was done to them, thus diminishing what little attachment was already there.
An animated series would be much more sensible. An average one costs about 1.5 million per episode, 2.5 if you want to make it look really good. They could use it as a series of short stories, kind of like the original Clone Wars series. Would help people actually familiarize themselves with champions as characters, not as eSports loadouts.
Second, Riot don't actually make any of the recent cinematics, they're outsourcing them to random studios all over the world. Bard's promo was done by Studio Mir, for example.