My guess is that it less far-fetched for the public to believe that physically fit humans will perform better in tasks in general, as opposed to say, a baby being able to differentiate between 78 and 79 dots on a flash card.
The other point, I suppose, would be that the study focused on 3-5th graders (8-11 year olds, respectively), whose brains (as well as bodies) would be more more comparable to adults. Given what is commonly understood of babies, it must be hard for many to comprehend how babies will perceive or react to stimuli (both physical and intellectual)!
shuki