Mostly by not-entirely-evil lich/mummies of their ancient kings. They still have living kings, but they tend to be not quite as important as the dead ones, because after a king dies he sticks around as an undead, and keeps all his followers and wealth. So mostly the living king gets used as a pawn by the more powerful houses of the dead.
Elves tend to have slightly more normal ruling systems, (often little or none at all) but there are also an unusual number of elven vampires and death knights running around, enslaving elves and mortals and building little fiefdoms. Elves also have an unfortunate tendency to come back as ghosts. It's just something that they do.
I'm not as completely sure about the elves as I am about the other races so far, but some one's ruled by undead. And since I don't know what else I might do with elves, it might as well be them.
This also sort of assumes 4e, but if I might end up using this for a setting that doesn't have eladrin. If so, I'll just stick it all together and say some elves are ruled by undead kings, and some are ruled by vampires.
As a further note, this is not entirely stolen from Eberron. Some of it is also stolen from the Incan empire, who mummified their kings and kept them around as rulers of political parties. Part of the reason they lost their war against the Spanish was they were already in the middle of a civil war between two mummies. (Not kidding, read 1491.)