Okay, I wanted to clear that up first because I've seen lots of grasp and DFT Mordekaisers get bodied and don't know why.
That said, Mordekaiser's kit is all damage. He has 4 high damage abilities, no CC, no mobility, and very limited utility (outside of his ultimate ghost as a pushing/objective taking force of course). He's very strong in lane and his all ins are absolutely deadly.
I obviously can't speak on behalf of Riot, but League is a team game, and a champion who's an absolute monster in the middle of a fight but has no means of getting into the middle of a fight is going to be depedent on his team to initiate. If Mordekaiser had the mobility and initiation to get into range in combination with the sort of damage to 2v1 (or more if ahead) his kit might have too few weaknesses.
Take what I'm saying with a grain of salt though, I play a lot of Morde with a high degree of success but #bronzelife.