Why Don't More CIOs Become CEO?
If you're C-level, you've got all of the corporate governance bullshit.
The real problem is that in most companies, the entire IT organization is seen as a corporate support cost center, like janitorial services.
Among MBAs, it is an old joke that CIO stands for "Career Is Over." If you're head of products, marketing or sales, you're seen as doing something "core" to the business and its value to customers. If you're CIO, you're seen as managing the IT department and technical product and service vendors -- supporting product development, marketing and sales.
Of course, in many large companies today, IT is the core value and a key driver of value to customers (e.g. FedEx). I might expect to see more CIO to CEO promotions.