Why was morde ever going to be the Lord of Death in the first place? Other than being a really, really powerful undead, he had nothing to do with the Ruination at all and he isn't really a primal force of death like kindred.
ModWulf Helhammer2 votesBecause he has power over the shadow isles, and was the first undead-ish person
BeatinWarDrumz1 votesBecause he has power over the shadow isles, and was the first undead-ish person
the first point is unclear - he is old, but not ancient. IMO it is likely that he is NOT the first undead. The reason he has power over the SI is because he is the most powerful person there, not because he was part of the ruination or created the Blessed Isles himself.
ModWulf Helhammer1 votesHe is ancient. His old kingdom is what the Noxus citadel is, and Nozus has been around a long time. And I never said he was part of the ruination or created he Isles, but the reason he is the most powerful being there is because he had already mastered death, coming back again and again when he was defeated in battle. That's how he has power over the shadow isles: because he wasn't created by them
Ebonmaw Dragon1 votesBecause he has power over the shadow isles, and was the first undead-ish person
The Kingdom of Mordekaiser is older than Noxus and Demacia, but there is a place older than them... 
SHURIMA...
And Shurima has an older undead: 
GreenLore1 votesUhm who said that shurima is older than mordekaisers?
It is easily possible that shurima(or at least amumu) came after mordes fall.
lightdragoon881 votesThere is one even older then Shurima and that is....Freljord!
The Howling Abyss. A bottomless crevasse surrounded by an ancient fortress carved into a glacier. No one remembers who built this place. No one knows its purpose. No one, but me.
Long before Ashe Ashe and Sejuani Sejuani, before magic shattered the broken peaks, before the desert swallowed Shurima, it was the time of The Three Sisters. We were so young back then; Avarosa, Serylda, and me, Lissandra.
Stars Shaper1 votesProbably because of how he resurrects each time and how he can manipulate the Black Mist and the SI inhabitants.
BeatinWarDrumz1 votes[{quoted}](name=Stars Shaper,realm=EUW,application-id=6kFXY1kR,discussion-id=7NBv2BKO,comment-id=0001,timestamp=2015-10-27T16:27:25.237+0000)
Probably because of how he resurrects each time and how he can manipulate the Black Mist and the SI inhabitants.
All of the other undead are unkillable... and manipulating the SI would make him the lord of undeath, not death.
Chromatic Eagle1 votesSeems like nitpicking tbh. Kindred are call the Eternal Hunters, not the Grim Reapers.
GreenLore1 votesthe other undeads are probably only intangible due to the black mist. Once they are put down for good,they stay down.
Morde however can always come back.