Until graduating from high school, nearly every young person in Japan will wear a uniform to school every day. During the course of an average day in Tokyo, one might come across thirty to three hundred young girls in uniform. Navy blouses, white blouses, three-stripe-style, anchor-style, navy skirt, plaid skirt, black socks, white socks, flopisox. There are maybe three or four general themes on which junior high(chuugaku) and high school(koukou) girls' uniforms vary.
By the end of the day, one has been taunted by the same unattainable bauble dozens of dozens of times. When one is in the market to buy pornography, the prospect of seeing (what might as well be) the same girl who sat across from you on the train or waited next to you at the crosswalk seems pretty appealing. There's a standard for what young girls look like, and that the girls in your magazines and videos match the girls you have lusted after in real life makes for a satisfying purchase. Probably.