Why is it that you only ever see sushi made with white rice?
Well, because of science.
Sushi, made with raw, boneless fish requires a delicate balance of flavors and textures that only white rice can maintain. Brown rice contains more of the germ that gives it its color, texture and flavor. The flavor of the germ often will overpower the delicate taste of fish.
Another issue is texture. There are thousands of varieties of rice, but two main ingredients: amylose and amylopectin. The balance of these key components determines the texture of the rice. Long grain varieties like brown or jasmine are high in amylose, which doesn't break down as much in the cooking process, and causes the rice to remain firm even after processing. This makes it a poor choice as a molding agent for sushi.
Many sushi fans prefer the taste of melt-in-your-mouth white rice which contains relatively little amylose, and it is for this reason that the use of white rice is all but mandatory even on today's culinary scene.