#the out of universe explanation is obviously that naming him in a mandatory cutscene would have given the entire thing away

[nemesis-is-my-middle-name]

i think it's really interesting that morgott names all of his "willful traitor" family in his boss fight cutscene and yet leaves messmer out, despite him seeming to be so overwhelmingly coded as The Quintessential Traitor. like, is it that

morgott considers messmer (again, heavily associated with snakes and fire, the specific symbolic enemies of the erdtree, and managed to get his whole throne removed) such a traitor that he refuses to even speak his name

morgott actually does not consider messmer a traitor (they do appear to share similar motives, and we have no real evidence of messmer participating in the shattering war, which is morgott's chief complaint about everyone else)

he's naming shardbearers who participated in the shattering war, specifically (godrick gets included) and messmer doesn't have a great rune/wasn't a part of the shattering, so whatever else aside he was irrelevant to the point morgott was making

none of the above; morgott doesn't even know messmer exists (very possible that messmer was born+grew up+got [???] while the omens were in the sewer and they just completely missed each other) so he's just like ah yes. malenia. miquella. that one inexplicable empty space. radahn.