Because of all the best known super heroes that have films made about them, Superman is probably the most fundamentally unrelatable. Batman may be a billionaire who has impossible ninja skills and technology, but he's still just a man whose parents were murdered. Peter Parker may have the strength of ten men and be surrounded by knockout babes everywhere he goes, but he still works at a newspaper for a cranky, abusive boss. Hulk has his anger issues, the X-Men are outsiders who have been cast out because of the way they are and so on. Hell, even Captain America, probably the second blandest goody-shoes character right after Superman has to deal with the fact that everyone he used to know is either dead or dying very soon and he doesn't recognize the world he's awakened in.
Superman has none of that. He's effectively indestructible, immortal, infinitely powerful, handsome, smart, can fly and always has the upper moral hand. He's not so much a character to relate to as an icon and an ideal to strive forward to, and havnig your character be like that from the outset doesn't really lend itself well to a traditional three-act story structure. Maybe that's why making him conflicted and have doubt was what they went with in Man of Steel. Granted, they could have done that less poe-facedly, but I still found the Superman in the movie interesting enough.
