I am genuinely curious. As of right now, they are the only characters able to interact with the terrain in such a way. The thing is, Skarner can only push someone a short direction after making contact, so what would happen if the push distance ran out while the two are still inside the terrain? Has anyone yet seen this happen?
When Skarner E collides, it checks a small area in front/behind the target to see if they're extremely close to terrain, so Kayn would get instantly stunned.
Kayn E is scripted to teleport him out if he ever receives a hard CC while inside a wall (IIRC).
Skarner exits the wall less quickly. His E buff drops, which makes him have to get out, which he does via a fairly quick pseudo-dash (IIRC).
Or they kiss.