It's not like Riot *can't* complete these stories. Just look at how much effort is put into the fairly new Ixtal region that was formerly either Kumugu or Plague Jungles. It seems it's definitely possible for Riot to finish up story plots from more than 4 years ago.
The Kai'sa one honestly left me speechless that Riot still goes down the "obvious mystery" road.
Just look at what they did with Burning Tides and Bloodline, Bilgewater and Shurima respectively. Please, bring some more light on these stories.