A moving target is less apt to get hit . Should a head of state vary his schedule/activities/itinerary so that no pattern emerges?
Their security detail is pretty good at protecting heads of state. But even they know, you can't stop someone who is determined to kill them and doesn't mind dying in the process. And since the 1950's, we have had to think about the possibility of a nuke.
But since the founding of the American government, there has always been a clearly defined backup in case the President is killed. And since the 1950's, there has been a permanent "shadow" government ready to take over in case a nuke takes out the President and Congress.
