Riot does a lot of things through the committee process (weekly votes within project teams, followed up by votes in larger groups and departments and a final vote of approval before release.) Champion design, for instance, involves teams with project ideas presenting their ideas before the group as a whole decides on who the next pack of champions will be down the line. Same with the skins team, champion balance, champion update and Narrative/lore.
Riot's also been, in recent years, all about getting the maximum impact with a minimalist approach. Bilgewater, for instance, only involved a quartet of champions to tell that story, despite there being three other champions with direct ties to the area (Fizz, Nami and Nautilus). As such, only four new intros/bios came out at that time. Another example was that Fiora had "surprise" rework done, so ChampUp and Narrative worked on her bio to time with that release.
So, instead of actually getting these intros done and released , especially for the 20 or so champs that are either so outdated that they no longer fit in the League canon (Singed) or no longer have a place due to the removal of the Institute of War, Narrative are trying to time the work with upcoming releases and so that these stories have the biggest reach possible.
That's why it's going to be another year before Yorick gets a new bio, or that we know what happens to Cho'Gath if he gets too big for Valoran to handle, or what the living hell Jax is without the League of Legends, etc.