Why is Sephiroth killing Aeris such a big moment in video game history

Probably because Final Fantasy VII is a popular game and it was just a major part in the game too. It would be the same as if Zelda died in the Legend of Zelda. Its a popular game series, and a main character. Her death would be kind of important. And probably now, because its such a terrible and unemotional scene because of the graphics so people like making fun of the graphics that were actually really amazing for its time. That is my guess.