Why do some organizations seem to have a new CEO every year or two, whereas others have top leaders who stay with the company for many years (e.g., John Chambers's nearly 20 years at Cisco)? What...

Why do some organizations seem to have a new CEO every year or two, whereas others have top leaders who stay with the company for many years (e.g., John Chambers's nearly 20 years at Cisco)? What factors about the manager or about the company might account for this difference?