TLDR; Because Neville didn't kill Nagini Voldemort was un-killable.
They state that Harry dies because Cedric Diggory kills Neville. This means the whole battle of Hogwarts is thrown off as now Voldemort still has his remaining Horcrux in Nagini, while we don't know exactly what happens, assuming that Harry and Voldemort still have the same show down in the woods, and Voldemorts Soul fragment is removed from Harry, they are no longer tied together anymore, and Voldemort or anyone else for that matter can kill Harry, while Voldemort can still be "killed" but has 1 horcrux left to re-resurrect himself with.
Snape: Cedric Diggory killed only one wizard and not a significant one — Neville >!Longbottom.
SCORPIUS: Oh, of course, that’s it! Professor Longbottom was supposed
to kill Nagini, Voldemort’s snake. Nagini had to die before Voldemort
could die. That’s it! You’ve solved it! We destroyed Cedric, he killed
Neville, Voldemort won the battle. Can you see? Can you see it?
Despite everything else the fact that Nagini didn't die throws off the entire fight, we don't know Exactly what happens but that is why Voldemort lives, and Harry dies.
Voldemort is not limited to Aveda Kedarva to kill Harry.
Since Nagini is not dead, Voldemort could simply use Nagini to kill him. Harry can still die, despite Voldemort being alive, Voldemort just probably cant kill him personally.
Its also not clear that Voldemort killed Harry, just that he is dead.
Have you swallowed something funny in there? Become a Mudblood without any of us noticing? Harry Potter died over twenty years ago as part of that failed coup on the school — he was one of those Dumbledore terrorists we bravely overthrew at the Battle of Hogwarts. Now come along — I don’t know what game you’re playing but you’re upsetting the dementors and entirely ruining Voldemort Day.