Meal has a point, Mordekaiser has built in weaknesses so he has to have something to compensate for being the only champion without any sort of cc or mobility outside of his W (which is teammate reliant to function as mobility in any case even in the best scenarios) in the same. Cc and mobility are there to help you get kills or avoid getting killed, Mordekaiser by himself lacks anything consistent in his kit beyond damage, shield, and two conditional heals.
As has been said, without building items like Rylai's Crystal Scepter or Hextech Gunblade, people could just walk away from Mordekaiser if they don't want to fight him, and the nerf to his base movement speed only creates a greater necessity to put more effort into covering his weaknesses.
Mordekaiser had Mercury Treads and Trinity Force for movement speed, and most likely there wasn't enough CC going around to truly pin him down. Also, I know Corki can travel over land very well with his Valkyrie, and Mordekaiser can catch up with aforementioned W if he has another champion chasing you down and there isn't more cc keeping Corki or Mordekaiser from chasing.
Yes, Mordekaiser can melt you if you end up in a situation where he can catch you, but Mordekaiser's kit is to punish people for making stupid mistakes and emphasize the need to kite him. Either you made a mistake of some sort, or you chose a rather immobile champion or wasted your cc in some way so that you couldn't keep him at bay.
...and the Corki you were fighting was AP, and had Rylai's Crystal Scepter as well, I honestly doubt between Corki's Valkyrie, Mordekaiser's Harvester of Sorrow, and both of them having Rylai's, that you could have escaped without some serious cc and mobility.
