The CEO is your marketer-in-chief: (s)he's selling how good the company is to your stakeholders, clients, and even suppliers (want a better, big, deal? ask 'em to talk with your CEO).
CIOs are not trained for that.
"The Avis WIZARD decides if you get to drive a car. Your head won't touch the pillow of a Sheraton unless their computer says it's okay." -- Arthur Miller