Too often in the past, we've seen them screw it up...by refusing to invest female action heroes with humanizing nuance, by writing them as archetypes instead of as people. Broad caricatures who have to lose what makes them women in order to compete on the same playing field as men.
Ultimately, it's because they didn't give Kal enough of Superman's personality. He was too much a cipher. A tabula rasa for others to write their versions upon. His motivations were ambiguous. Why was he wandering around for years?