 Why don't the cousins realize the flaw in their plan?
No one, least of all Martin, could dispute that a man's life should be guarded by his kin—but by those who hadn't been born yet? "He's dissatisfied with the current social order and killing you is part of an elaborate plan he's formulated to change it. There was no devastation, no war, no unhappiness, none of the concomitants of normal living. It was then that Martin began to realize that either the whole lot of them were insane, or what Ninian had told him at first was the truth. And, on those worlds, it's far cheaper to use native labor than to ship out all that expensive machinery.
 They are not very intelligent.
 They do not understand time travel.  They all originated from the same point in time.  They are highly interbred.  They are not very intelligent.