@Riot, I'm not sure if you have answered this but it's making me wonder

Because they didn't do them when they made his VU(around that time they started to make random splash art updates based on the preferences of the artists,which is why we got stuff like the new brand splash art or the new count vladimir) and once they started doing splash art updates they did some others first for several reasons: was done because her Riot-girl skin is the first skin for a lot of players was done because her splash arts happened to be almost complete from the time when the process was unorganized because he was among the first champs to get a big overhaul in terms of backstory and theme and there was a huge difference between his new and old designs was done because she barely has any skins who needed a new splash art(I think it were only 2) was done because there was just this huge difference between his designs. was done,because just like Trundle,she was among the first to get a big VGU.

and will be the next ones as they are part of the upcoming Tank update.

Overall I believe that karthus is fairly high on the update list as his new design is also quite different from his old one,but the others simply needed them as much/even more than him,or were simply lucky(Mao and Sejuani wouldn't get their new splashes so soon if it werent for the tank update)