Children are young, innocent and widely appealing. School uniforms invoke a sense of nostalgia and make people living in Japan's harsh working culture think of

Children are young, innocent and widely appealing. School uniforms invoke a sense of nostalgia and make people living in Japan's harsh working culture think of their much happier high school days. Plus, much of a child's life will be spent wearing one so if you're depicting a child it's a realistic choice of clothing. As for why girls over boys, I think it's that cute and innocent things are appealling and more people find girls cute and innocent than find boys cute and innocent, even the artists themselves, regardless of gender. Many artists have mentioned that they find girls easier to draw than boys as well. There's also the sexual aspect but though I think such media is more common in Japan than elsewhere, I don't believe it's anywhere near mainstream.