Ainsworth remembered that Gabriel Cross's betrayal had caused a global commotion, not because it was very rare for humans to betray Earth. But due to the traitor being one of humanity's most promising prodigies at that time.
To prevent any future prodigies from going down the same path of treachery, the Global Alliance and Awakener Association went all out in assassinating Gabriel Cross, and they succeeded.
As for why Ainsworth remembered a dead guy like him, it was due to his sister being somewhat in the same year as Gabriel Cross and even fighting against him.
Like him, his sister Kyla had also represented Edenbridge High, and she still won against Gabriel Cross as well as against another Rank 1 prodigy from Valebreak Citadel. However, she admitted that if not for her existence, Gabriel would have been the strongest in that school year.