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?
Christina Hendricks has the old-world, but never old, sex appeal of our Marilyn, and is perhaps our first red-headed pin-up since Rita Hayworth. And she has major acting chops.