Because he isn't a hero, that's why not. I don't care
what happened to Khan after Marcus woke him up, that doesn't change who Khan really is - a tyrant. It doesn't erase Khan's actions during the Eugenics Wars.
To make Khan a hero would be to excuse what he did back then. Why would anyone want to do that? Don't the Eugenics Wars matter? Why doesn't Khan deserve to be punished for what happened then? Why should we just forget about them?