Cook was left out of the one day side because it was felt he lacked the instincts to play aggressive shots. He took umbrage at that decision. No doubt that he's a top opening batsman tho but unfortunately he's the only one currently that can open.
Trying to find someone to open with him is a real issue for England. From memory - Compton, Trott, Ballance and Jennings have all struggled in one way or another. There maybe others that I've forgotten as well.
Any team will struggle if they find themselves 2 or 3 down before lunch.