It's just too heavy for Superman to lift

Superman isn't invincible. He has his weaknesses. In fact, Superman's great weakness is probably about as well known as Achilles'.

But when Superman died, it wasn't Kryptonite that did it. When Superman died, it wasn't his tragic flaw, or his great weakness that did him in. Superman died when his strength failed. He just met someone stronger.

There's a lot about the Death of Superman storyline that I didn't like (particularly his re-incarnation as an energy being), but the means of his death is exactly right. Kryptonite may make Superman weak, but to actually kill him with it would seriously impovrish the story. From a mythic standpoint, Superman can't die because of his weakness or because he fails somehow. Superman is not a literary tragic hero. When he dies, it is simply because he finds something too strong for him—too heavy for him to lift.