I can give you an anthropological reason. It comes from thousands of years ago when humans, (and human ancestors,) were not just hunters, but also prey. We had to be able to react to an ambush from an attacking animal, so we are actually better able to detect quick movements out of our peripheral vision than dead on, as many attacking animals pounce from a hiding place in order to catch their prey.
c&p from www.goanimal.com
QUOTE
bio-evoIt's easy to understand why peripheral vision would have been preserved in human evolution. As a prey species, humans have long had an urgent, compelling need to look around their world and monitor the periphery. Survival in a mosaic grassland would have required a comprehensive, wide-angle visual scan. Dangers can be anywhere: left or right, above or below, in front or behind. If you focus in one place for too long, you become vulnerable to attack from some other direction. Ideally, you'd have two overlapping sensibilities; one to focus on the center of attention, another to monitor the neighborhood. Given the demands of their environment, we can assume that primal humans used their vision in a comprehensive manner, a rough balance between peripheral and focused vision. They focused on objects of attention of course, but they also kept a broad scan going. They concentrated on food, friends and other curiosities, but they always maintained peripheral sensitivity. This is the default use of vision, the natural use of the human eye.
(I know, I'm a big dork, but I love this stuff!)

eta: OT, but I'm all excited! I'm an appairiton and not ectoplasmic residue anymore!!!

(I know, I'm still a dork.)