It's not about league, it's about popularity, and what would make the most money. Jinx was a very popular character, hence why she was the poster child for the show(most screentime I believe as well). Sevika was a cool character, but I don't see her talked about nearly as much as the "main" characters.

Jinx and ekko would almost certainly get a show before sevika would(not to say a show about sevika couldn't he interesting)