Everyone’s wanted to be Superman at some point in their lives. Then we grow a little and learn that a hero is better when they’ve got problems, hurdles and other things that normal people have to deal with, like heartburn or paying rent.
Superman is always good for a quick result, like that friend who's able to give you a ride to the airport on short notice, but it is true that the vanilla version can get a bit stale after a while.
It could be, perhaps, due to the character's somewhat overpowered nature. Others would probably mention just how gratingly all-American he is, or even the character's sometimes deontological boy scout tendencies. Whatever it is, readers have often found the character lacking and writers have done plenty to reinvigorate the Man of Steel. Most if not all attempts have, however, just killed the character off.
Other attempts have createde alternate versions of Superman, both of Kal-El/Clark Kent and the actual Superman identity itself. Over the Man of Steel's eighty-four year tenure as a comic book icon there have been so many alternations of the character - some better than others and some just so bizarre you probably haven't even heard of them.
Who'd have thought there could be so many Supermen?