Bear in mind that:
(1) The role of CEO of the world's most valuable technology company would exhaust most, completely fit men. Fortunately, Tim Cook is up to the task!
(2) Steve is probably recognising the fact that after such an extended period of time as acting CEO, Tim would be under unreasonable pressure having to report to someone who might not, from day-to-day be available.
Hopefully, this decision will give Steve a greater opportunity to manage his health...