That's the thing though. League, in general, is suuuuuuper expansive in the lore. We can assume there are at least 130+ stories operating at the exact same time, with some overlap. While in theory a base League show would be great, there's just a ton of lore content that couldn't possibly be covered, and then some fans would feel left out.
In the case of the Star Guardians, however, there are at current 10 Guardians, with the possibility of more. They have a tightly constructed narrative that focuses on the 2 small groups and the people within it, rather than 130+ different narratives that have little to no overlap in a lot of cases. It is much simpler from a storytelling perspective to tell a story about the Star Guardians than it is about all of Runterra at once. There's less characters to focus on, and therefore each character can have more meaningful screen time, and because there's less characters, there's more opportunity for better storytelling, as the writers will have sharper focus.
Sure, people outside of League may enjoy the series, but why would that be a problem? If they like Lux the Star Guardian, they can still like a show focusing on her, and they may indeed be tempted to play the game, if it's their thing. If it isn't their thing, well, Riot has enchanted an audience that the game didn't reach with an aspect of their lovable characters. It's nothing but a win-win for everyone, if we can get a fun show.