What has been discovered by survey upon study upon investigation is that poor kings live longer, ...
This isnt that odd actually, since a good king is a major threat to your opponents, making it profitable to assassinate him, while a bad king is, in a way, a good asset for your opponents.
In my current Muscowy/Russia game, Ive had all the first five rulers die prematurely sending me into a regency council each time, grr. And, of course, my current ruler is an idiot, so I expect to be stuck with him forever.