Actually they once clarified why we see these splash art updates not that often anymore.
When they started updating splash arts, they didn't really have a set direction for it and pretty much gave the artists free reign over which splash they want to update.
But this changed once they decided to ship VGUs with splash art updates. So instead of updating random splash arts, they update the skin splash arts of reworked champions.
And since the number of VGUs increased quite a bit over time, they simply took up all the budget for splash art updates.(I mean this year alone we had ~20 completely remade skin splash arts + some that were adjusted for the VGU)
The reason why we even got some additional splash art updates after they started updating them alongside VGUs was because they had some almost-finished skin splash arts from the first phase lying around and decided to finish these, since it wasn't that much work in the first place.
So its not that they put less resources into new splashes, its just that they are tied to VGUs nowadays.