Like if you blindly asked someone "There is a skill that lets you mark a champion and if they die you control their soul for a while. Who does it belong to?
And the choices were the master of metal and the lord of death 100% of people would say the latter.
That has more to do with offensive skills designed to kill things being under the wide umbrella of the incredibly vague word death than anything else.
Teddy bear of death.
Fire of burning death.
bullets of death.
dunkmaster of death.
Death fits pretty much any skill that does damage.
Also, Mordekaiser's ultimate inflicting pain and damnation is still fairly Heavy Metal (along with his physical appearance).
More importantly though - a title should say something about a champion that's unique and worth pointing out.
Mordekaiser is a master of Heavy Metal with a lot of heavy metal themes, which makes his master of metal title at least interesting.
Master of Death tells us absolutely nothing about the champion, due in large part to the aforementioned vagueness of Death as it pertains to a game where you're trying to kill one-another.
Mordekaiser's name also translates into Death King, so calling him the death master master of death is pretty redundant and derppy.