WRT Superman, Marty, and Doc. Time Travel is a bitch.

1. Multiverse theory in which they don't actually prevent it, just move to a timeline where it was already prevented. Those people still die in your original universe (and an infinite number of others), you just don't see it because you've moved to one of the infinite numbers where they don't.
2. Grandfather paradox. You can't go back in time to kill your grandfather as a kid, because then you would cease to exist, and thus, be unable to kill your grandfather, which means he survives to go have your parent, who has you, who goes back in time to kill grandpa, who doesn't have your parent, who doesn't have you, so there's no one to go back to kill grandpa. Basically, something will happen to either prevent you from stopping it, or find a workaround to have it still happen (even if slightly different).
3. Butterfly effect. We don't know how changing something like that would affect the rest of the timeline.