There's usually a 1 patch delay between compensation and a change so the effects can be measured
and so the changes don't seem revolve around that one champion skewing the win and play rates as everyone and their mother floods ranked and blind pick with mordekaiser.
If Mordekaiser got some sort of buff the same patch Rylai's got nerfed there would be an annoying amount of new players lowering the win rate of Mordekaiser. By changing Rylai's only and letting Mordekaiser be the same, Riot uses the very basic principle of a controlled variable and lets Mordekaiser play with only those changes having an immediate impact, the play and ban rates and average playerbase games remain fairly similar for one patch where the Rylai's change is in effect. It's also why follow up tends to be one or two patches away once the buff hits, play rates change as the patch notes bring attention to the champion and Riot waits for it to die down.
Anyways Mordekaiser mains should be glad as being overtuned in the same patch they get a nerf means they're lumped into the Syndra, Yasuo, Riven, etc club.