From a 1991 essay, “What purposes can “international terrorism” serve?”:

…I want to offer another conjecture on why international terrorism is so rare…whereas individual acts of terrorism may be easily within the capabilities of quite ordinary individuals , a sustained campaign on any scale may require more people and more organization than could be viable in most target countries. And there may be some negative feedback from the low success rate to the low attempt rate: Resourceful individuals, people with brains or people with money, may find terrorism so unpromising that they do not choose to contribute effort or money. And any organization that is secret and dangerous risks both defection and infiltration; a group of people large enough to carry on a sustained campaign, perhaps simultaneously in different target areas, may simply be too vulnerable in defection and infiltration. Even seeking financial help risks being informed on.