Because power corrupts and absolute power corrupts absolutely?One would assume continuing to have the adoration of the people(by being the chosen successor of the Benevolent/Enlightened Ruler), and the lifestyle that affords you, would be enough. I am not sure as to why it does not work out that way more often.
I can see one person learning how to rule benevolently, and still surviving a while. But that is the freakish exception to the rule. So two in a row isn't likely.
Tom