Do you really have to ask? The reason the US has so many great 5k and 10k guys and so few good half marathoners is because PED's work best at 5 and 10k for men and 800/1500 for women.
Here's another puzzler for you guys: Both Japan and the US have had established distance programs for decades and if you look at top lists from years back you realize the Japanese and Americans always did about the same in the 10K. The top American typically about 27:30-40 and the top Japanese 27:40-27:50. Suddenly in 2009 the Americans start running sub 27, sub 13 and the Japanese still remain at 27:40. Of course there is a perfectly plausable explanation for American improvement: Better nutrition, better coaching, the fact that American runners before 2009 were mentally retarded, smoked cigarettes and ran 20 miles a week. So why haven't the Japanese made the same type of improvements? isn't there a Japanese runner that can afford an altitude tent? Why hasn't there been a genius Japanese coach who has discovered that his/her athlete needs to focus on closing speed and open up the floodgates to sub 13 and 27? Are they just lazy and dumb?