About twenty years ago, probably in a hope to shore up falling sales, DC comics ran the "Death of Superman" story in which, as the name implies, Superman dies. This was a huge event in comics, so huge in fact that it actually made the mainstream news.
Unfortunately, it also ruined everything. Well, not everything, but it certainly did no good for comics or for "Superman" movies.
The fundamental problems with the "Death of Superman" storyline are two-fold: firstly, it's such a huge story that it has come to overshadow everything since; and secondly, it's just not a very good story.
The primary influence of the DoS story on the films has been seen in two of the three "Superman" films released since that storyline: "Superman Returns" features a moment where the world isn't sure if he is dead or alive (spoiler: he's alive), while "Batman v Superman" takes that storyline as one of its two primary inspirations ("The Dark Knight Returns" being the other). And if DC were to ditch their current plans for a Cinematic Universe and reboot the character again, you can be sure that pretty soon they'd find themselves doing "Death of Superman" again. Because every Superman story now lives in the shadow of this one.
But "The Death of Superman" just isn't a good story. Basically, Doomsday appears, being a thug with no personality, no motive, and no backstory, and proceeds to lay waste to things. Superman faces off against him, the two spend numerous pages trading blows, and finally Superman kills Doomsday. But he then dies from his injuries. And that's basically it - the whole thing is basically just a way to get to "Superman's dead", with no character development to speak of and only the barest hint of a plot.
And so we find ourselves in a position where any cinematic version of Superman must address this story and this story just sucks. That's not exactly a good position to be in.
But it gets worse. Because the "Death of Superman" wasn't the end. Of course it wasn't! It took a little while, but that story is inevitably followed by Superman's return from the dead - and, in the DC universe, the opening of the Revolving Door of Death. (Which, coupled by the constant need to reboot and update means that nothing can be permanent in comics. Any character who 'dies' will be back in a couple of years, if Peter Parker gets married that whole arc will be retconned out of existence on a whim, and so on and so on. Nothing sticks, so nothing can have a lasting impact, and any impact it does have will inevitably be muted.)
In the short term, the "Death of Superman" storyline was interesting and probably had the impact that was hoped for. In the longer term, though, it really has ruined everything.